2011 heater/blower noise and vibration solved. "Prius goes nuts!"

Purchased 2011 Prius 2 with 108500 miles on 4/29/2015. Didn't fully check all climate control settings and fan speeds. Two days later, car had vibration and rumbling sound which was increasingly severe on three highest climate control fan speeds in all modes (e.g. AC, heat, auto, manual, etc.). Sounded like a quiet lawnmower on highest fan speed. Was quiet on three lowest fan speeds. Glove box and passenger side dash vibrated in sync with noise.

Cabin air filter had been replaced by a large Toyota Dealer as part of pre-sale prep. I suspected filter was not installed properly. Removed glove box and filter, which had been properly installed. Vibration/noise were same when fan was operated without filter or glove box installed.

Felt around inside of fan housing. There was debris, including seven pieces of acorn shell (see image below). The concave sections of the shells were firmly wedged in the fins of the squirrel cage (no pun intended!) impeller. Cleaned out all nuts and debris, reassembled, fan operated quietly without vibration at all fan speeds.

The shells essentially unbalanced the impeller and induced vibration at high fan speeds, just like an unbalanced wheel would induce shimmy or vibration at freeway speeds. What is baffling is how the debris and acorn shells this large got past the filter. Filter fit on the right side is pretty good since a plastic flange goes between the pleats. The left side appears to have a looser fit with some gaps.

Lesson to be learned: Don't expect the cabin air filter to trap all debris, clean out your squirrel cage when you change the filter!