Primary grinding. A constant grinding sound can be caused by a misaligned or loose chain as it tries to re-cut teeth to accommodate its path more easily. I don’t even want to get into all the damage all those fine cuttings will initiate.

Also, a new chain should be broken in and checked after 500 miles. Consider changing oil as the break-in period generates lots of filings. No lubrication causes all kinds of havoc and noise and is evidenced by an orange rusty color evenly dispensed throughout the primary,. The chain must be inspected for a broken or missing (disintegrated) shoe that can emit a grinding noise.



Source: http://www.heavydutycycles.com/techtip2.htm