Listen for noises when the engine is running

There shouldn't be strong noises, coming from the engine under any condition with no matter is the engine cold or hot. By the way, knocking or tapping at a cold start is one of the indicator of poor maintenance. Knocking, tapping or rattling noises indicate excessive wear of internal engine parts. Whistling may be caused by loose belt. If the engine makes too much noises, avoid buying such a used car (however, Diesel engines are always more noisy, it's normal).