I agree with Shooter98. Don't overlook the obvious.
- loose bases or anything about the scope that may be loose
- properly torqued stock/action screws
- anything touching the barrel
(my sendero had the fore sling swivel stud rubbing the barrel)
- clean bolt and trigger mechanisms
- changes in ammunition or shooting conditions (have a buddy shoot the rifle)
- does it have a muzzle brake
and does the problem persist with/without the brake
- are your bolt lugs gauling
What's your cleaning procedure?
- use a bore guide and run your rod only from the breach end
- use a carbon fiber or non-abrasive/plastic coated rod
- bronze brushes (never stainless... especially on stainless barrels)
- powder solvent
- copper solvent
Hope this helps!