2004 Mazda Mazda6 - Loud belt tensioner


I recently had a new belt tensioner installed after the old one fell off, and now my car is making a sound like something under the hood is running on overdrive (maybe the belt?) It sounds like a motor running way too fast.

It’s loud, and doesn’t seem to matter if I’m moving or at idle. It also continues for many seconds after the car is off (which it never did before).

Any clues?

why not bring it back to where you had the work done being it happened right after they supposedly fixed it.

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I would, but I’ve since moved away and looking for a new mechanic. I just want a clue about what’s going on so I can weigh that against a professional diagnosis.

if they just changed the tensioner and belt it could be another pulley thats on its way out. with the extra tension of the new tensioner pulley and belt its putting more pressure on everything else connected to the belt. but you say it happens when you shut the car off? the only thing that might run after the car is off is your electric fans that cool the engine


That would make sense. When you fix one thing you often end up chasing problems around the car.

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Not a mechanic, but it could be a bearing making a new noise due to the new load it feels. A mechanic will have an opinion on if that is good or of no real concern. The after-run has me stumped. Please circle back and tell us the outcome.

Yes, very strange. Almost wonder if it’s a sensor or computer issue.

Raise the hood and see if one of the radiator fan motors is noisy.


Not likely, noises typically are mechanical issues, like worn or bad bearings.

I wonder if a vacuum hose got knocked off when the belt tensioner was replaced. This would explain the fast idle. If you mean that the engine keeps running (which we used to call dieseling) a few seconds after turning off the key, a disconnected vacuum hose would also explain that problem.


An update for anyone following: Took the car to the mechanic and they determined it was a bad fan control unit causing the fan to run all the time. Ordered a new unit and will probably just replace in myself.

Thanks for following up.