torqueisfun

I have a 2009 C63 with about 44k miles. I just started getting the CEL P0171 for Bank 1 System Too Lean. It appears that this could be an intake manifold gasket leak, bad O2 sensor, or a couple of other things.
I am also getting the code for the Bank 2 System Too Lean so it doesn't appear restricted to one side.
I have also starting hearing a fairly loud whistling noise when I shut off the car. It almost sounds like air whistling. I cannot hear anything with the car running. I had my wife start and shut off the car a few times with my head under the hood but I couldn't quite pin point the location of the sound. The high pitched whistle only lasts about a second or two, like something is releasing pressure.
Any ideas, all help is much appreciated.
I have a recent Weistec tune but other than that, the car is stock to the best of my knowledge. I've owned it for about 7k miles and I've had the tune about 3 weeks. No issues with it for the first couple of weeks after the tune so I don't think it is tune related.

Mort

Sounds like the intake manifold gaskets are leaking to me. There is still vacuum in the intake when the car is turned off and you are hearing air being sucked into the intake through a small gasket leak. My 0.02.

torqueisfun

I am leaning towards that and the PCV valve (breather pipe) based on some other threads I have been able to find. Looks like a little DIY is in order.

AdonisVA

Was your issue resolved? :-)

torqueisfun

Yep. Had to replace the PVC valve. Cleared it right up.