When I stop at a stop light or a sign, a second or two later, a knocking noise under the car begins to start. It is not rapid but a slow knock. It shakes the car when knocking. When I put it in park, the knocking stops. It basically knocks when I apply the breaks. Can someone please tell me what the problem could be and how much it would cost to fix it.
Thank you all.

The exhaust system is very well known for lots of various noise patterns. IN fact, mine has just started acting...again...a few days ago. Check all brackets, and exhaust heat shields that use to fall down and trash the road....

Well, my exhaust end pipe fell off, but that was a few weeks ago. This noise occured just yesterday. Do you think it is because of the end pipe?

The tail pipe IS a part supporting the muffler alignment....
....the tail pipe end bracket had rusted away on mine and was the recent culprit for the noise I mentioned above....