Strange noise from a Toyota 4Runner

So, I have 100K miles on a 2007 4Runner (V6) and after having my radiator flushed at the Toyota dealership service dept, I get my car back and now hear a low rumbling noise coming from the engine compartment. I hear this noise every time I turn on the air conditioner, and sometimes even when it’s off.

I took my car to Toyota today and after getting the service advisor to acknowledge the noise, I get a call 2 hours later saying, a) that no noise was identified by the technician; b) nothing wrong with my car; and c) I can come pick it up.

It ain’t my imagination. Clearly, something is wrong. Can you tell me what it is? Should I take it to another service department? This kind of thing makes me crazy!

Thanks for your advice.


Is the serpentine (accessory) belt original? With 5yr/100k miles it wouldn’t hurt to replace it. An old belt can get pretty noisy.

Ed B.

Hi Ed,

Yes, it’s original. The engine itself sounds like it’s running fine, but it’s this “accessory” noise whenever I turn on the ac. Does the serpentine have anything to do with this? Is replacing it a big job?


Just a guess, perhaps one of the electric cooling fans got wet with coolant during the flush process and was damaged and now is noisy. One fan kicks on and off with the AC unit. The other 2 fans go on and off based on a sensor detecting “hot” coolant. The tech doesn’t know your particular car and how it normally sounds, so an unusual noise to you is within normal for the tech.

This is not your imagination, but it is hard to tell if the noise really indicates a problem or not.

UncleTurbo, thank you for this thought. I had the AC accessory belt checked and was told it was in fine condition and tight. So your explanation makes a lot of sense and explains why this began after I got my car back from the radiator flush.

I’m wondering if this fluid, if it did get on the fan, will burn off and quieten down. Hopefully it’s not damaged, but it does sound rough.

How about air in the cooling system. Is it a constant gurgling sound behind the dash?

No, galant, it’s not a steady noise, unless I turn on the ac. Then it’s this low, rough “idling” noise. It does sound like some small motor behind or beneath the engine. It comes on when the ac is on, and sometimes even when it’s not on.

I think I’m gonna try running the ac as much as possible to see if I can get the coolant to burn off (if this is what happened, like Uncle Turbo suggested) and go back to sounding normal.

Just wanted to leave an update, as I appreciate the comments and advice/feedback left here regarding my problem.

Not long after my previous post, I took my 4Runner on a long road trip and at some point during the trip I noticed I no longer heard the noise from before. So I believe Uncle Turbo hit it on the head. I think some of the old coolant spilled onto one of the fans and when the Toyota dealer flushed the radiator, and this coolant eventually burned off.

So far so good. I have 136K on my 4Runner and it really has been a dependable and trouble-free vehicle, this noise aside. :slight_smile:

Thanks again to everyone who responded to my question.


Could have been a little air in the cooling system too. A small amount can bleed off by itself, ends up in the overflow tank presumably. If this happens again, were it may car, the next AM I’d park it in the driveway, with the radiator end of the car higher than the other end if possible, remove the radiator cap, and let it idle until the thermostat opened. 5 minutes of this in most cars would usually get the air out of the cooling system.