Help! Strange sound when accelerating
I've been getting some strange noises when accelerating lately. It's kind of an ocsillating sound, not the knocking-type sound, nor is it constant... but it sort of sounds like a sound that peaks then goes away, and it cycles through until I get to higher rpms. The lower the rpm, the more pronounced the noise is, and it's not noticable at higher rpms (over 3500-4000). The sound actually gets louder as the engine warms up, and more so if it's cold outside. It's not very noticable to begin with, but it's not something I've heard in the car before, so it's a cause of concern for me.
Some other information:
- 2008 328xi e92
- I cannot notice the sound if I push the gas in while the car is in neutral. This would leave me to think the sound is associated with the drivetrain other than the engine, maybe the wheel bearings?
- I've had the lifter ticking recently, and my engine is pretty loud when I lift it up to hear the sound after the car's warmed up. Could the oscillation effect of the sound that I'm hearing in the cabin be related to this?
- I've recently had an oil change. Possibly just subpar oil?
- I've recently noticed that my left rear brake clip is missing. I think it's very unlikely that this would be the cause of the sound, but is it possible?
- Car had 2-3 rough starts in some fairly cold weather in one day. Was back to normal the next, albeit a little bit warmer the next day. Maybe bad gas/oil?
- Still under same battery since 2008. Probably has nothing to do with it but just throwing it out there.
I'm still under CPO, but since they don't fix anything unless you are sure something is wrong and can prove it to them, I'm afraid that I'll be left with a massive bill on my hands after the CPO warranty expires.