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What's that ticking sound?

Good question!

On this week’s show, we heard from Rich in Denver, who’s got a '96 Chevy pickup with a ticking noise coming from a box under the passenger seat.

Could it be that his Chevy’s self-destruct mechanism is finally kicking in? Tom and Ray had a few less exotic possibilities, including antilock brakes. Which his truck doesn’t have. Or a power antenna. Which – you guessed it – his truck doesn’t have, either.

Got any ideas? Share them right here – and thanks!

Hey guys,
I think that this may be a obstruction in the blower for the a/c-heater unit. Rich mentioned that he was getting out of the truck because it was hot! Hence no air being circulated. I do not own a Chevy pickup nor do I have first hand knowledge of that make of truck so the truck may not be the only thing full of hot air.

Great Show,

Never seen anything like this but…

Its not a “ticking” sound, its a “thumping” sound like a dog’s tail hitting the dash. If you listen very carefully, the first thump takes a bit longer to occur than the metronomic precision of the the rest of the thumps. Anyway my vote is the blades of a fan hitting the paper folds of a cabin air filter.

Good Luck Rich

Dear Tom & Ray,

I have a '98 Chevy truck and that 10" dia black box under the front passenger dash is the heater blower - right up against the door pillar. I bet he’s got the heater/ac controls on and the fan is trying to run but hitting something and never getting up to full speed which is much faster than the frequency of his tapping noise. I don’t have a power antenna but I expect that mechanism is under the fender, like my Jeep. I agree the frequency of his noise is similar to a malfunctioning power antenna.

Great show!!!
Peter Ross,
Bolton, MA

I have an '93 Chevy that I think makes that same noise. I asked my dad(an ex- GM mechanic) what he thought the noise was, & he said it was either the fan motor, or the flapper door actuator that’s supposed to open when you set the blower to the floor. So I replaced the fan (it needed to be replaced anyway). That didn’t fix it, so I told him, and he then suggested the flapper door(above the passenger floor, to the right[it’s near the door];[there might be one on the driver side{I didn’t ask him}]). He said he doesn’t know exactly where it is, because it depends on the year of the truck.
I hope this will help you out. I know the noise is annoying, but you’ll get used to it(I did), until it gets fixed.

P.S. My dad also said that it won’t affect the trucks performance at all.

The ticking is the acurator motor. Its a black box at move a a.c. door for max air. It makes this sound when you turn the key and when you turn the truck off. The motor part is working but the pastic gear that more the door is bad. The part is a dealer item and they much the # off the top of is black box.

Good Show,

I’ve had the same problem with a '93 Chev PU and a '94 Chev suburban. It’s the A/C recirculation door actuator motor which is electrically driven on these vehicles. When it fails it makes that thumping noise as it tries to actuate the door – there is some timer that eventually times out and the thumping noise stops.

PS. If I remember correctly, there was a lady from Idaho that called Car Talk with a similar problem several years ago.

HI guys, I believe his air damper control module is malfunctioning.

ok i had the same problem, its a striped out gear inside the box. what it does is when u turn on the key it opens a door inside the air duct & when u turn off the key it is trying to close it, so there u have it. rob from bartlesville oklahoma

Wow this was the exact question I was going to ask. I have a 1994 GMC 2500 and it does the very same thing. I have had a friend look at it and it is the box that is making the noise. Mine does it when you start it and turn it off. the only thing that was not mention is that the air conditioning does not work well when the fan is put on high.
Gail from Bend, Oregon.

I have a “91” Chev. That noise is coming from the bottom vent underneath the fan motor. If you force that vent open, the noise will go away. It’s annoying but it’s not hurting anything and not worth repairing. It lasts 23 seconds and then goes away. You have to trust me on this, mine’s been doing this for 4 years. Glad to hear someone else has this problem. My daughter nicknamed the noise “Thumper”.

Thanks, Bob

Our 94 Chevy PU had the same problem. Electric repair shop replaced A/C “bland door motor”, & it works fine ever since. Cost in 2005 was $140 labor & $66 parts.

Good luck!

Yep, Striped blend door actuator, here is where you can get one.,carcode,1031498,parttype,10721 click on more info to see if thats what it looks like that is making noise

Amazing consequence. I was listening to Tom and Ray this morning. I pulled into the cleaners and immediately a knocking noise (never heard before) starting from under my dash (?94 Suburban). I guessed it was something to do with the A/C control door because when I pushed the MAX (inside) air control it stopped for a moment and then started up again. It now continues to knock for 20 secs. after the ignition is turned on or off.

Amazing part is that when I returned to the truck, T and R were just starting to answer the call from Rick about the very problem! Could my Suburban be psychic??

I also think the problem is the blower fan is hitting the shroud.

I have a 94 with the same exact problem and I found out how to fix it. (and am currently still living it until I tear into it)

CLICK AND CLACK are you listening??? You guys missed this same problem a month ago when a guy called!!
Talk about stump the chumps.

It is a faulty RECIRC door servo motor.

It seems like when you turn the truck ignition on or off the AC control unit has a prove out circuit that runs to verify the position of the recirc door.
The system acts like there’s a limit switch that died and is not telling the AC control box that the RECIRC door is shut and then it proceeds to send power
to the servo for its maximum time limit.
You can make it click again whenever the vehicle is in operation by hitting the recirc door, because once again the door hits the end of its travel and the
AC control box doesn’t know it.

With all due respect to the other faithful Car Talk patrons, their explainations of fan obstructions, fan shrouds rubbing, vacuum motors (Tom and Ray!!!) and other stuff is just silly if you have HAD a vehicle with said problem… (as an aside after 88 Chevy used electric servos to run the doors and not yesteryear’s vacuum motors)

Take it to the bank…Bad Recirc door servo motor.

Thanks guys
the Lunatic has left the building.

Hey guys,

I had the same problem on a 92 GMC pickup and a 94 suburban; It was the actuator for the recirculate fresh air air door, located behind the glove compartment and behind under the dash. Broken tooth on one of the gears. It can be removed and replaced without tearing the dash apart if you get a offset ratchet (spec tools, squeeze wrench) Americar posted the location for an online source which was less than half the $92 I paid at the chevy dealer.


I don’t have a chevy pick up, but rather a Ford Windstar and had exactly the same sound coming from behind my glove box. A trip to the Ford dealership revealed that several pieces of papers from the glove box had slipped behind the box and were blocking a mechanism that is supposed to move the AC air flow from fresh air to cabin recycle. I’ve noticed that you already have several other posts to that same diagnosis, and we are probably all right.

FYI - 4 wheel Anti-lock brakes have been srandard equipment since 1995 and the problem is most likely an A/C problem

It is probably something stuck in the squirrel cage in the blower fan. Maybe a dead mouse or chipmunk spinning and slapping the inside of the housing.