Hello Brickboarders,

This is an informational post to help those looking for an answer to a
buzzing sound that occurs under the hood after you shut off the engine. It may alternate 10 seconds on 2 seconds off...or advance to long runs with short breaks if you've ignored it for some time.
It comes from a cylinder shaped item buried below the air box on the left front of the engine compartment.
It is from the Brake Vacuum Pump.
http://www.fcpeuro.com/products/volvo-brake-vacuum-pump-s60-s80-v70-xc70-xc90-hella-31317445?gclid=CPm3tOfT2bsCFbFxOgod7DQAyQ
I also replaced the Booster Vacuum Switch.
http://www.fcpeuro.com/products/volvo-booster-vacuum-switch-s60-v70-xc70-s80-genuine-volvo-31265825?gclid=CNuVi5LU2bsCFag7Ogodg20A5g

I had let this go a long time (> 1yr) and the runs of buzzing pump grew longer and would last 30-60 seconds...I stopped counting. Several with short breaks.
I changed out the Booster Vacuum Switch because it was easily accessible and that changed the buzz to rapid short bursts that ended after a minute.
Then I spent the time to work my way down to the Brake Vacuum Pump and replaced it. No growling buzz. I bought it used for $40 on ebay and took a chance.
If you have a growling buzz then change out the pump. I can't say whether I wasted $60 on the Booster Vacuum Switch but I'm not going to bother swapping the old one back in. After all it gave me 200K service and the new one totally changed the demand on the growling pump.

If you let it go indefinitely I suspect that one day the pump will fail and you might not have Brake Vacuum at spec before the engine has a chance to produce it. I've seen other forums where people have referred to the system as over-engineered. I would disagree.

Analyzer
2003 V70 220K
1997 850R 139K
Retired/Sold/Totaled; 244, 245, 744T, 965, V90