SOURCE: 2006 volvo s40 50,000 mi.
The oil filter housing also houses the pcv system. What causes the noise is that there is a check valve in the pcv system that is supposed to regulate the pressure in the crankcase. The valve gets stuck and causes vacuum to build in the crankcase, which in turn sucks air wherever it can causing the noise, which is also why when you pull the dipstick it stops. This can also cause a check engine light with codes for fuel trim. The oil filter housing will need to be replaced. Good news is that Volvo extended the warranty on these to 100K miles so it should be free of charge at the dealer. One thing to watch for is oil leaks after the replacement from the camshaft seals. If air can get past a seal, usually oil can too. If the seals leak too long it can contaminate the timing belt and cause it to break, which causes a lot of damage.
SOURCE: noticed a noise under hood that sounded like a
Every car has a carter recirculating system: in the carter there are gases that escape the cylinder during compression; if they would be left there they would in the end explode on their own causing damage; because of that they must be extracted simply by sucking them out - a pipe is connected from the carter to the admission manifold and the depression there sucks the gases out; the gases must be replaced by something so air from the outside (usually thru the oil cap) is sucked in its place - this is what you hear.
Absolutely normal operation, at worst the air hole is slightly blocked by dirt and the noise is way louder that it should be.
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