grinding or scraping
say like when you check your clutch for slipping 6th gear at 40mph and nail it to watch the tach itll make the noise.
it sounds like its coming from the bellhousing area, but i could be wrong.
i doubt bent pushrods could cause that either, since its only at low RPM with more than 1/4 throttle or low RPM more than 1/4 throttle and a higher gear.

