Strange noise when starting to accelerate
 

2012 bmw x5 35i e70 has 100k miles and a not so loud, but loud enough to notice in and out of the car, noise when idle or beginning to accelerate. It is springy in nature, a higher pitch, a little bit like a rattle. I was thinking like a rocker arm/ valve type noise on an older car. Or somesort of gearlike thing engaginh and disengaging in a cycle. When idle it is intermittent and doesn't change with heat or ac engaged.

It doesn't seem to affect my performance (but is a little embarrassing) and goes away as soon as I accelerate from a stop.

Theories or guesses to what itt could be?

wastegate?

If it happens at idle can you figure whereabout on the engine it is coming from? Internal? external? if you can take a short video of the sound with the hood up and post it to YouTube it would really help in diagnosing the issue.

I'm not sure what a "springy" sound is? I'm thinking of a cartoon-ish "Boing" which doesn't mesh with the rattle/tapping of lifters & rocker arms.

Process of elimination. Try to isolate it first and go from there. Give us updates!

Belt pulley?

Is it more apparent under hard acceleration, compared with gradual acceleration?

Definitely under slower acceleration but it only lasts for a second or two once the RPMs get above 1000, doesn't matter if it's in Park Drive reverse or neutral, then it's gone. I've tried to record it on my phone but my phone doesn't have the proper format to upload on to this website.

And sometimes it doesn't happen at all.

I do hear it when I decelerate from driving to a stop when the RPMs drop from above 1000 to below. There's no noise at idle around 700 RPM

Without more information or a video I'm inclined to think something in your serpentine system. The RPM correlation leads me to believe a pulley bearing. With bad bearings, they tend to have a "sweet spot" for making noise.

With your attempted video recordings could you at least isolate whereabouts on the engine it is coming from? Stick your head in various parts of the engine bay and you may be able to isolate where the sound is coming from. This may make a world of difference in diagnosing your problem.