A few years ago I had this crazy high idle that would have me zooming down the street at 30 mph without touching the throttle. I replaced the throttle body with a used one which remedied the problem.

Recently the problem has returned but it's not nearly as bad - it mostly just idles around 1,500 rpm for a few seconds after I come to a stop. I (supposedly) separately have a P0400 code that pre-dates the high idle by a few months.

I watched a rabbit-hole thread of videos on youtube trying to figure this out. I found a video where a tech used a shop glove over the MAF inlet (removed the engine cover) to test for a vacuum leak and then turned the engine over for a few seconds to pull a vacuum. If things are sealed, it should hold for many seconds. His held until he pulled the dipstick out. A side note: my dipstick doesn't seal inside the tube like his did (Mercedes M112).

When I did that, I had a big hissing sound around the throttle body/MAF and the vacuum dissipated within a few seconds. The MAF doesn't seem to be terribly solidly mounted on the elbow (yellow arrow below) to the throttle body which could be a source of the leak but I don't know if this is normal or not. And I don't know that this is where the hissing sound is coming from.

Any ideas?

Thanks.