1992 Buick Riviera

I recently bought a 92 lasabre and it has a cracked radiator in it when I fill it with water and drive it to the store it makes a thumpin noise in drive, but compleatly stopped. When I hit the accelerator a little it goes away, when I put it in park the noise gets softer and sounds like from the rear of engine. It needs a full tune up. What do you think the noise its?
May 29, 2007.

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