I may not shed some light on your problem but it sounds a lot like the noise that my new Mitsubishi utility (6,500 klms) with a 3litre V6 engine made. Took it back to the dealer and let the service manager hear it and initially when I said it sounds like 'piston slap' he said no couldn't be. Well with the bonnet open and the vehicle stationary he ran it up by pulling on the throttle cable and at certain revs the noise seemed to resonate very loudly and it was definitely from the bottom end. With an incredulous expression on his face he goes away to fetch the big boss service guy and they both come to the conclusion that yes something major has happened in the bottom end with the result that they pronounced the engine U/S and that they would replace it with a new short motor from the Manufacturer. Took a week and now the new engine is smooth as silk (as opposed to the original which now seemed to always have a slight 'rough' edge about it). (Who said Volvos are unreliable?)

Good luck with your problem - it doesn't sound like anything simple.

Cheers, Phil

'98 NA S70 129,000 Klm