Timing Chain rattle?

[Unique X5]

2005 Turbo diesel with 172,000 k's driving normally in low revs the engine is very smooth and quiet, especially for a diesel. When I accelerate hard and from about 2500rpm it makes a rattle sound very much like a lose timing chain hitting the cover. Anyone with a diesel tell me if your car is quiet at higher rpm, as diesels are normally noisier at idle, or if noisy like mine? Also free revving it in neutral it doesn't seem to make the noise as I would expect it to. The car doesn't blow any white smoke and it does not sound like detonation (pinging).