Two problems related to manifold/header/muffler/tail pipe have been reported for the 2011 Toyota Prius. The most recently reported issues are listed below.
Loud rattle, shaking in motor area. Was found to be a head gasket leak. Using a lot of antifreeze and white smoke out the tail pipe. While idle or in motion and while gas engine is running. Started intermittently at first and very short, eg. For a second or two, then got louder, more often and lasted longer. This car has 128,000 miles. This same issue is happening on my 2010 Prius with 250,000 miles. I researched this and it is a common place with gen 3 Prius cars. I can no longer drive either one of my Prius cars safely.
Upon ice start up at times, there is a loud knocking noise emanating from engine compartment. Goes away after 10 seconds and car drives fine. Has occurred several times. Though there is a TSB issued, dealer would not fix unless issue is replicated despite showing them what my car did on video. Most noticeable after car was run for a very brief time hours before. Also notice the last few weeks, while driving vehicle, upon ice starting up, the engine is rough and the shift to ice is noticeable. Previously it was smooth. No codes have appeared and the car appears to run normal otherwise. I would like the dealer to fix the problem as described in 2 separate tsbs (engine mounts and intake manifold gasket).