I'm not an alarmist but I had a simular thing with a first year Volvo S80. Deep thumping knock much like a rod knock. Every time I took it to the dealer it had gone away. Told them to keep the car until they figured it out and only try under a cold start. They found it was piston slap. They said about 2000 engines were built with excessive piston clearance. Got a brand new engine at 30k miles. Problem solved. Don't ignore it! Funny thing, my S 80 was built the first year Ford owned Volvo.