M112 Piston slap on warm engine?

[volvo164]

Hi
I purchased a 2002 CLK 320 recently. It has a knocking sound that starts once the engine is warmed up, and at low RPM, with a slight load (like being stopped with the transmission in D or R). You can hear it even in Neutral or Park, it's just not as loud. Sometimes it's louder. I've replaced the oil with Mobil 1 high mileage 20/50 synthetic, with no help. A local shop looked at it and said it was piston slap, but said it should be OK. I'm surprised because I thought piston slap occurs on cold engines, and goes away once warmed up, not vice versa. The sound is definitely coming from the engine block, not the valves, and it's not pre-ignition. It's a clear metallic clanking sound. Any thoughts on this?
Thanks, Laszlo