When i take foot off of the gas car knocks!

Just a few hours ago i noticed that when i took my foot off of the gas my car knocked twice…At first it was just 2 quiet thumps and i thought it was the brakes…But after driving more this evening the noise (i dont know what to call it…click thump or knock?) got much worse. It knocks every time i let off the gas…I am obviously worried…any ideas?