My 2011 BMW 335i coupe convertible is making this noise and I have the check engine light on the dashboard. I took it to the mechanic and did a compression test, and he told me he's getting low numbers due to an air leak. Anyone knows the cause of this issue or facing the same situation?

[VXEric]

Probably PCV. if you open the oil cap and the sound changes, that's what it is. pretty common issue with the N55, you can search for a lot more info.

My good friend (a mechanic) and I opened the oil cap in his shop, and the oil spilled all over the engine, it took us like 1 hour to clean. We drained the oil, and put new one. We brought in a mechanic that worked on BMW's for 30 years, and still didn't diagnose the problem. We found an oil leak from the pan when we lifted it on the jack, and checked with a flashlight. So my friend is suspecting it's either a bearing or the gasket inside the oil pan. Would you recommend doing a smoke engine test to see when the air leak coming from?

[Rob09msport]

If you have low compression and you are pressurizing the crank it sounds like motor is hurt. The air leak is the crank venting ,bad head gasket could cause low compression as well. Do you know what the readings were and was it across multiple cylinders? If all cylinders are close to being equally low that could be pointing to a different issue but unfortunately pressure in block shpuldnt cause low compression but blow by can cause pressure in block