1996 Chevrolet Express 1500 / G10 - Thumping/Chopping Noise During deceleration

chevrolet
express1500

#1

I have a 1996 Chevy Express 1500 van. RWD with 5.0. During light braking or deceleration (probably around 30 - 35 mph), it makes what I would call a rhythmic thumping or chopping sound. Though it is hard to describe a sound in words, the best way I can describe it is that it sounds like fan blades striking something. I realize there is a cooling/radiator fan on the vehicle, but I would presume this van would must likely be on only at idle.

A shop mentioned that this thumping was coming from the crossmember (which I actually did not even think my van had) and that someone had forgot to replace a bolt when installing my catalytic converter. I am not sure what to make of this, or how I would be able to verify it for myself. It seems like “crossmember” can mean different things to different people.

Any advice would be appreciated


#2

How come this shop didn’t fix it?


#3

First thing I would be checking is U-Joints and center carrier bearing


#4

That is a good question. Unfortunately, I was not actually the one who the van to the shop and did not get to speak to them; I was was just reading the receipt/diagnosis notes from the shop. When I got the vehicle back, I asked my sister (she was the one who took it for me) if they fixed the “thumping” problem, and she replied that it sounded as if they did (or thought they did) fix it. Apparently not. They gave me an estimate for other work they thought the van needed (fuel pump, fuel induction, MAF sensor cleaning, etc.) but no mention of anything relevant to the thumping sound.


#5

Good point. Is there any good way one can check those without having the vehicle on a lift? I would rather check it myself if possible, but I don’t have a lot of equipment, or, admittedly, experience with suspension or drivetrain components.


#6

If it is running well the items the shop thought were necessary probably are not.
Did the thumping start soon after the cat replacement? If so, I would take it there first.
But either way I would try a different shop that the one that wants to do the fuel system service.


#7

If you want to do some investigating underneath, raising the vehicle onto ramps is a lot easier and a lot safer than jacking it up and supporting it on jack stands. Never, ever get under a vehicle if it’s supported only by a jack.

Apparently some part of the exhaust system is sometimes sagging and touching a crossmember. Once under there you can wrestle with and knock on things and see what you can see.

I am not impressed by a shop that says there’s a missing bolt and tries to sell you unrelated fuel system services.


#8

I don’t know how much ground clearance you van has but just driving up on some pieces of 2x4 will probably give you enough room to slide underneath. Grab the driveshaft and try to rotate it. If you hear a clunk or feel a lot of play, there is your problem.


#9

Thanks everyone for the responses. I am still investigating. Just to clear up a bit of confusion, I actually took my van into the shop for two primary issues: the thumping noise and missing/surging under load. The fuel pump was their solution for the miss (even though the diagnosis ppears incorrect - I replaced the fuel pump and the issue persists), but they did not seem to really do anything about the thumping issue other than the remarks mentioned earlier.

Anyway, I do not have a stable way to jack or lift the van, and I live on a hill. However, I a can fit underneath the van pretty easily. I saw the hole for the bolt the shop was presumably talking about being missing, but not sure how it would cause the noise I was hearing. It is one of three that goes into a metal bar (on the passenger’s side) around the area of the catalytic converter. I could not move the driveshaft nor did I notice anything unusual around the U-joints. I did notice that there appeared to be some buildup/grease around the rear differential, but it has looked that way probably since I bought the van, over a year ago. I could move the exhaust a bit, starting around the muffler area, but the noise in question sounds like it is coming more from the front of the vehicle. Otherwise, I did not see anything out of the ordinary.

I appreciate all the comments so far, any additional guidance would be appreciated.