2004 XC70 Whirling sound only when NOT accelerating

[Elisa]

Hello all,

I am having an issue with my 2004 XC70 w/230+ mi. I am hearing a very audible whirling (not quite whining?) that started very suddenly the other day. It only seems to be happening when I am NOT accelerating, with the exception of going up an incline, then I can hear it faintly. I can instantaneously make the sound stop and start with the slightest touch on the gas. The faster I am going with no acceleration, the higher the pitch. When coasting to a stop, the lower it gets, with or without use of brakes. I am feeling no pulsing or knocks. I do maintain this car pretty well and fluids are checked regularly.
Side note- while driving home the other night I was praising to my husband how well my car has been running and gave the dash a little thank you pat. Not 1 minute later this sound started happening!!! Did I jinx It???

Thanks all and looking forward to any theories...

[Elisa]

Additional info:
The only thing we have done recently is replace the fuel filter as it was leaking. Today I also replaced the air filter thinking it might be an air intake problem, but after driving, no change. Our working (uneducated) theory is bearings in one of the gears on the serpentine belt is failing and that with acceleration it has more umph, but with no acceleration it is struggling. Also read some forums on breather box problems but that is way over my head...

[abscate]

Hello ForgerofMetal

Use a hose in your ear as a stethoscope to track down a noisy pulley in the engine compartment.

I going to guess is the alternator pulley, which “ free wheels “ on these cars; it isn’t just a solid pulley, it has a clutch. You might be able to hear the change while WoodWorker revs the engine slightly.

[Elisa]

Thanks @abscate! We were about to try this suggestion but worse things happened
I took it down the road to see how it sounded today and it had a very rumbly hard start. When I started driving, hardly any acceleration. My generic code reader gave me P2111 - throttle actuator stuck open.
I did leave out one thing. After the initial problem, we popped the hood and what I think is the vacuum pump switch (right in front of the air filter compartment with an orange clip) was not attached. We put it back on with the little rubber gasket thing inside and secured it with a zip tie. Is there supposed to be a spring in there too? I think I read that somewhere...if so it is gone now.
Could that have been the start of this all?

[abscate]

Oh yes! That vacuum switch tells the brake vacuum motor to pump to give you power brakes. When they fly apart, it probably is telling to it to run at bizarre times.

This also fits your symptoms thst the pump whines under acceleration when vacuum is low, and you might be hearing it now with the switch broken?

Cheap fix from FCP, about $70for the new, improved design switch. 314000693

https://www.fcpeuro.com/products/volvo- ... escription

[abscate]

I got that symptom backwards to your report. As Roseann Roseannadanna would say...

never mind!

Replace that brake switch, then try the stethoscope trick. Don’t leave that brake switch too long, it might be running your vacuum pump constantly which will wear it out, and it’s not cheap

[famfeld]

Most likely the valve for the brake booster is the culprit and you need the plunger and the spring. Can't remember if they are together or separate.

[Dhillon]

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