recently got a 2000 sls, 89k miles now. I have no prior experience with these motors. i get a kind of hollow sounding knock; it is different than anything ive heard before. not really like the thud of a rod knock and it is not at a steady rate either. one more weird thing about it- it does not occur while the engine is at idle or while the engine is revving up.. only when it is slowing down or held at speed above idle. sorry if that is all just ridiculously vague but i wanted to see if anyone has seen something like this before i start tearing it aprart

Check the accessory bearings as well as the idler & tensioner and remember, the water pump is on the back (drivers side) of the engine and also has a tensioner.

Steering pumps can make engine knock sounding noise.