Space Shot "thunk"

Has anyone else noticed that the SS side of PT makes two "thnuks" every time one of the cars launches? Since it happens every launch, it probabby isn't a big deal, but I'm still curious. To elaborate more, it does a big thunk about half way, then again on the way back down only softer. I havn't noticed the TD doing that.
The only thing I can think of is the air valves, but if that is so, why doesn't TD do it? Or is TD just very soft?

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Millennium Force - world's first and greatist gigacoaster!

It is air pressure at the top. If you notice, when that car gets to the top of the tower it doesn't exactly run out of speed, it has to be kind of thusted back down, it happens close to the top.

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Number 1 And Only Cedar Point.

I think you're hearing a "quick exhaust" valve. To allow the air pressure in the cylinder to push the car up quickly, it has to let the air out of the other side in a big hurry. On the pneumatics on smaller systems, it's usually a rubber diaphram that opens to release the air. They make a "pop" sound (you can hear it occasionally on Mean Steak's brakes when they release). On Power Tower, the thing has got to be much larger, which would make a lower frequency sound, or a "thunk".

i always thought it was where the pressure went from positive to negative and then back again

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please ignore my many typos and grammar errors :)