Volvo Troubleshooting Questions? Ask a Mechanic for Answers ASAP
if by pulling out the dipstick the whistling noise stops then I'd say that there's a fault with the PCV valve
these are sprung loaded and are only meant to 'burp' excess pressure every now and then. The exiting gas is piped back to the inlet, however should the valve stick open then the engine tends to suck the air out of the crankcase all the time and this carries oil over with it.
If you remove the air intake pipe and look inside where the crankcase breather is plumbed in you may find evidence of oil.
If you do find oil replace the valve.