My 1988 dx sedan makes a rattling noise under the drivers seat when the gas pedal is released and the car starts to coast the noise is especially evident at higher speeds. The noise sounds like a large washer is loose and rattling around. Does anyone know what this is?

 

take a look man
it really is that easy

is the noise inside the car or outside
then look

 

Try rocking the car back and forth while it's parked to see if you hear the rattle. Could be the piping from your exhaust system loose. Is the much rust on the bottom of the car?

 

first thought was exhaust piping
Check your engine mounts
Check your exhaust hangers