Sterling recently did my warranty replacement turbos and did a fantastic job. It's all about the SA, ask for Tony Cardenas. I actually got a diagnosis from bullet for the wastegates, brought it to Tony, and a week later I had two new turbos, a new cv axle boot, blower motor, and right passenger mirror, all covered under cpo warranty, with no headaches, just a $50 deductible. Even had a fun 128i to play with in the meantime. Look at reviews of bmw dealerships, sterling is pretty high compared to the others in the greater LA area.
South bay is actually the one currently wasting my time diagnosing my a/c compressor for 2 weeks, couldn't possibly recommend them, extremely rude and pushy staff to boot!
And I live near Santa Monica....I won't even go near that dealership...
BTW, I've never heard of an air filter replacement being covered under warranty. Sounds like some kind of misunderstanding.
There's no reason to deal with the wastegate rattle as long as you have. Go with the route I suggested or a similar one and they will be replaced. I bought my car a month ago from Rusnak Thousand Oaks (Terrible) and thought it was normal at first until doing some research. Thank god it still had 4 months left on the cpo warranty. Had the repairs started at Sterling within a few weeks of buying the car, when after installing a cobb ap I started throwing underboost codes. Took it to bullet to diagnose what I thought was a boost leak. Instead was told right away it was my wastegates. I dropped my car off at Sterling the next day. Now I'm hitting 18psi and running a 4.6 0-60, so I'm quite happy
The catch 22 of course was that the cobb ap would void the warranty if found (it wasn't), but at stock boost levels I didn't trip any fault codes, so without it, I never would have gotten the turbo's replaced under warranty....