Oscillating drone from engine at idle

keaunym · Post by **keaunym** » 24 Oct 2016, 08:45

I'm trying to isolate the cause and remedy for an oscillating drone sound coming from the engine of my new/used S60 I picked up last weekend. It is an '09 2.5T with 82,000 miles. It makes this oscillating drone sound at a rate approximately once per second and a half at idle. It can't really be heard while the engine RPMs are up and the vehicle is driving, although while coasting at low speed, I can faintly make out the sound. This sound is not associated with any change in RPMs. Oddly, parked at an incline (rear higher than front end) in my driveway it would reproducibly make this sound while the gear lever is engaged in Drive, but not while in Park. Parked flat, it will make the sound regardless of the position of the gear lever. Any ideas?

Post by **vtl** » 24 Oct 2016, 09:07

Replace: upper engine mount, transmission torque rod, front and rear engine mounts, right engine mount - https://www.fcpeuro.com/products/volvo- ... p2mmkit2p5

Strut brace mounts, left and right: https://www.fcpeuro.com/products/volvo- ... ht-8666204 https://www.fcpeuro.com/products/volvo- ... ht-8666205

Car will be quiet again.

Hutchinson is OEM for these mounts.

keaunym · Post by **keaunym** » 24 Oct 2016, 09:31

vtl wrote:Replace: upper engine mount, transmission torque rod, front and rear engine mounts, right engine mount - https://www.fcpeuro.com/products/volvo- ... p2mmkit2p5

Strut brace mounts, left and right: https://www.fcpeuro.com/products/volvo- ... ht-8666204 https://www.fcpeuro.com/products/volvo- ... ht-8666205

Car will be quiet again.

Hutchinson is OEM for these mounts.

I can see how engine noise and/or vibration could be caused by bad mounts, but is the oscillating part of the sound typical for bad mounts in these vehicles? It sounds like a purring cat as the sound gets loud, then soft, loud, then soft...

Post by **vtl** » 24 Oct 2016, 10:44

Yes. If you hook up DiCE to the car, you can see that idle rev is actually oscillating a bit.

Other source of oscillating sound could be VVT system, though usually it kicks in when VVT solenoid(s) change angles of the camshaft(s). You can temporarily turn VVT off by unplugging VVT solenoids.

keaunym · Post by **keaunym** » 24 Oct 2016, 13:42

Do you mean to say that there could be a problem with the VVT system or just that this is a consequence of normal VVT function? For what it is worth, I just drove to and from lunch paying attention to vibration (not sound) and noted that the actual vibration that could be felt from the engine seems minimal and what actual vibration can be felt, feels steady and separate from the oscillating sound. I'll try again feeling for this oscillation with my hand on the engine later and report back.

Water pump, rad fan, gunked up engine... Could any of these be likely reasons? I had a mechanic that specializes on Volvos tell me based on description that it was likely a dirty throttle body but I failed to mention to him that RPMs don't fluctuate. Could a dirty throttle body still be the cause of the sound even though RPMs don't change?

Post by **vtl** » 24 Oct 2016, 13:48

I had an oscillating vibrations in the range of 1450 up to 1650 RPMs on 2002 V70 with N/A engine, which caused by a worn VVT hub. Vibration gone with a new hub installed.

You have a Bosch throttle unit, it is barely breaks or reacts to a gunk. If it does not close well enough at the rest position, you'll surely notice unevenly idling engine.

Did you ever do a PCV glove test?

keaunym · Post by **keaunym** » 24 Oct 2016, 14:45

Ok, glove test comes back normal. Vacuum is applied to glove instead of positive pressure. The sound appears to be coming from a belt pulley in the timing belt/serpentine belt area. Whatever it is, it turns on and turns off too, so there are periods when it doesn't do. If I time it, it purrs for 20 seconds, then stops purring for 7 seconds, then does it again. The timing is reproducible. All at idle at 600 rpm.

Post by **vtl** » 24 Oct 2016, 14:48

Disconnect VVT solenoids and see/hear how it goes.

keaunym · Post by **keaunym** » 24 Oct 2016, 14:56

Ok, will try later..

keaunym · Post by **keaunym** » 24 Oct 2016, 19:35

Ok, I had to look it up but I found that the plug is under the plastic cover by the timing belt. Please confirm. Also, is this the only plug I should disconnect?, should I do this with the engine off, then start?, and should I expect to see a check engine light that will go away after I reconnect after shutting engine off? Thanks in advance!