Lots of things to check in buying used car, better bring a trusted mechanic with you, under carriage noise is easier to resolve than unusual engine noise so becareful, if you want to check combustion efficiency of a car, check the tail pipe, it must have no oil, and in the morning when you start up the engine for the 1st time, if water condenses out of the tailpipe they say this is a good sign as to engine combustion, i'm sure there are lot of other things you should check, so research as well...