I couldn't tell easily from your description, is the vehicle in motion when this is happening? If the knock is coming from the engine internally, you've got an oil supply problem. Perhaps oil in the crankcase is not being picked up by the oil pump - a filter element may have failed one time and clogged the pick-up screen - or your oil pump is worn out. It's not a situation I would let go without investigation though.

2002 X5 3.0 257,000 miles
2004 325i 114,000 miles