Update:

PHEW! *wipes forehead*

The rattle was the result of the plastic cover over the distributor rubbing against the blower fan. Apparently it shook loose during the really rough start experienced after letting the cleaner soak.

Car is running better than ever; no rough idling, no hard starts -- all is well.

Also, the oil level was noticeably lower after the process but when checked again this morning, it had risen back to normal. (no explanation)

Sorry if I alarmed anyone, do not be afraid to use this cleaner... it DOES work.

~~Paul