Ha! Good to see.

Long story short, I just installed a vortech blower on my car, and two days later when I'm at a stoplight I had this same thing happen to me. Scared the hell out of me. Went home, checked every single hose clamp, bolt, and soldered wire, and found nothing. I still thought it had something to do with my engine work, but... this is it.

Doesn't tell me what it is, but...

 

Happened to me yesterday. Wife and I got in the car after a tax prep appointment so the car was warmed up. I started the car and a few seconds later there was a thump noise under the drivers floorboard.
Wife said, "what was that?" I said... "who knows"

 

LOL!!

my 09 GT 5spd does the same thing!!!

Feels like pressure relief of some sort. Mine did it to me and it felt like a shock or strut popped back into place. Only has happened when starting up. Once after a 30min drive to get my daughter. I got out, and when we were leaving it popped! Scared the crap outta me too. and I have less than 1320k on mine.

About 4x's now and Im now up to 1320miles on the car only

 

So no updates from anyone, huh?

 

My car does the same sort of thing, but not only on startup, it will do it a couple of minutes after startup. I only notice it sitting still at stoplights and mine sometimes occurs more than once.

 

My car has 70 K on it now about, 12 K with the blower. This still happens but I haven't felt any ill effects from this.Except that it still scares the crap out me every time. lol

 

mine 2004 GT did that on and off for 4 years (40,000 miles). and my 2008 does it every once in awhile. i have never had a problem from it, so i basically ignore it.

 

OMG! I have been having this same issue! I only heard it twice, once I thought i imagined it and today--no imagining it!-- it was loud!.. just a couple of days ago I had my A/C charged and also had my transmission fluid flushed(no real issues with gears grinding or anything , i just figured it had never been done and it prolly could use it)... I was about ready to freak out until I read this. My Mustang is a 2000 and Ive had it since October and never heard that sound before..( i usually have music blaring and I havent lately).but this sound is, as others have said, not just audible but can be felt. I am so relieved to read that about the a/c clutch and the CASS... Thanks for all the helpful info!
~ Sherry