Are you sure it is not the timing chain for the two cams? There is a chain between the two cams with a separate tensioner. I've never heard of that tensioner going but might make knock sound if tensioner failed.
Most common thing is the guide rails to fail, at a 99 car is probably in the correct mileage range for them to go. I think knocking sound is very obvious when they go. Timing chain tensioners is a don't drive situation. Test is to remove the sump pan and check for broken bits of plastic in the sump.
An e39 is like a beautiful wife, needs constant attention, lots of money, lets you down at the worse times but you’d never be without it.