The car is a 1994 940 non-turbo (belongs to a friend).

The engine idles fine with no unusual noise.
Under heavy load it runs fine and makes no unusual noise. For example I can put the car into 'D', foot on the brake, floor the accelerator, and there is no unusual noise.
However, letting my foot off the gas causes a terrible knocking from inside the engine, until the it is back to idle speed. It is a very loud metallic noise and sounds like a hammer banging on the block. It is definitely not piston slap.
Driving at a slow constant speed around the block causes lighter banging, but again, as I let my foot off the gas and the engine slows down, that's when the banging is by far the loudest.

So there are the symtoms... idles without noise, runs at load without noise (or very little noise), but decelerating causes a loud banging noise.

What's the problem? Thanks in advance for all opinions.