Recently, my vacuum-actuator for the supercharger's bypass valve stopped working (stopped holding vacuum, basically), resulting in the SC never being properly bypassed.

I have the 5th gen (intercooled) MP112 Magnuson supercharger on my LS1 engine (and I imagine, for the sake of SEO, that it's the same setup that goes onto the LS6 and other LS engines, among others.)

The resulting rattle and other issues were annoying - if you have an MP112 and you start hearing a metallic rattle at idle, observe whether the bypass actuator's metal ... hook-pole thing ... that pulls down the bypass valve is the thing that's rattling against said bypass valve.

You can also easily tell if it's not working by opening the hood while idling. With the throttle plate nearly closed, the engine should be pulling a high vacuum, and this should be pulling the actuator down, so that the SC is in bypass mode. If it doesn't immediately pull down when the engine is started, get a new actuator.

Their website is pretty hard to follow, but apparently the right part number is 45-00-13-113, and on their (fairly poorly formatted) website, it is listed as the actuator for the 4th gen. Well, they confirmed it was the right component for my car, sent it over (at a cost of about $150 with taxes and shipping) and it fits fine and solves the issue.