Running the heater makes the whole car shake
 

Today when I turned the heater on, the whole car started to vibrate/shake. I turned it off, and the shaking stopped. I turned it on and off many times, and sometimes the car shook, other times it did not. Another interesting point is that when I turned it off, it seemed to take a couple seconds for the HVAC system to "spin down," and the shaking/vibrating was at it's worse during the three seconds between the LCD saying the fan is off, and actually hearing the system stop.
Has anyone else experienced this before? What could it be? Loose bolt on the compressor? Please tell me this is an easy DIY fix.

on auto? AC compressor runs on auto mode...my car's RPM's dip for a second when the HVAC kicks on after starting

Sorry, just trying to decipher if it is a engine thing or a fan thing. Does this happen with just the fan? Engine off, fan on, can it be reproduced?

I'm guessing motor mount.

Thanks for the responses, guys. The motor mount was replaced a few months ago. It is definitely related to the compressor. I keep the HVAC on while turning on and off the compressor. It only shakes when the compressor is on.
I'm guessing the compressor is just loose or something. Where is that? Is that easy to get to?

remove the panel behind the seats, then you can observe what is happening.

Fantastic! I was hoping that would be the answer. Thanks, tommy986.

My guess is that the problem is in the idle control - its not properly adjusting for the added load of the A/C.

When you turn the fan on in Auto mode, the A/C compressor comes on and extra load is bogging down the engine idle to where the car shakes. Turn off the fan (and thus the A/C) and the idle resumes to normal in a few seconds.

I finally got around to taking the interior engine cover off today. Everything there looks good and tight. There seems to be no play in the compressor pulley or anything else.
thstone, things seem to be pointing to what you suggested, the idle control valve. That said, the car has always and currently idles perfectly. Knowing that, do you still think it's the ICV that is the culprit? If so, is this the procedure you recommend?
Clean the Throttle Body
Thanks all!

I wonder if your condenser fans are going when the compressor cycles on. If the fans do not go on to cool and lower the pressure in the a/c system, it is very hard for the compressor to compress a high pressure gas. One of my Boxsters had a dead compressor when I bought it. Not hard to replace though.

Hmmm...interesting. How would I diagnose that?