Loud buzz in engine compartment

[wgriswold]

Lately, after turning the engine off there is a loud buzz coming from the engine compartment. It lasts for 15-20 minutes or so and seems to come from the area under, or near, the air filter box. It is fairly loud and difficult to isolate so I am not completely sure about the location.

Any ideas about what this is?

[howardc64]

I know of only 3 electrically controlled valves around air box that can probably buzz. TCV (attached on the air box), ABS is under it but towards the firewall, and EVAP purge valve (buried near bottom of airbox for an 03 I think, 05+ is under intake manifold near #5 cylinder)

Use a tube from a paper towel roll or long screw driver to see if you can isolate it.

[pbierre]

Here's a few techniques to localize an abnormal vibration sound.
• e-stethoscope: wearing earbuds, probe around engine using cell phone's mic (e.g., voice memo app)
• old school stethoscope: use 1970s era airline headset; touch components suspected of vibration
• vibra-stick: use handheld aluminum chopstick poking around to feel for source

[wgriswold]

Thanks. I will take a closer look.

[perkinscl]

I had the same symptoms you have. It ended up being the Brake Booster Switch - Volvo Part # 31265825. It's a switch next to the airbox. Disconnect the wire harness, if the buzzing stops, that's it. Simple to replace.

[AutosDirectFlorida]

Its not a fan sound is it?
I believe there is a computer fan of some sort that runs post shut down - mine does it from time to time but genuinely sounds like a fan.

[Willy]

I never heard of that before, only the engine cooling fan.

[Astro14]

Best $20 you'll ever spend:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

It has both a sound tube, and a metal probe.

Use the metal probe to check components. The noise will conduct very clearly through the probe. You'll know what's making the noise when you find it.

[MacNoob]

I had the same symptoms you have. It ended up being the Brake Booster Switch - Volvo Part # 31265825. It's a switch next to the airbox. Disconnect the wire harness, if the buzzing stops, that's it. Simple to replace.

Yes, this. The brake booster vacuum pump (under the airbox) is quite loud. The switch is in front of the airbox and has a wire harness and vacuum hoses connected to it.

[wgriswold]

Thanks everyone for your replies. I was gone for on a trip and when I returned the buzz was gone.

I suspect it will return and when it does I will trace it down, with all your good advice, and report back.