I agree that it is a mechanical issue - but it is common to both. Have you serviced both engines prior to running them? If so look at what you may have disturbed by accident while doing so.
If not, then try a lub and grease job and see if the issue disappears.
Get real close down at track level and very carefully look and listen for changes when the issue appears. You may have a binding issue.
Try both on your test track and try to figure out what is common to both that would cause this problem by using the technique above.
