C6 noise

Maybe someone can help?
I have a Ford C6 on the back of a 351C in a 1973 Iso Fidia (big Italian Sedan 1968-74).
After its first drive in months (less than a mile) I noticed for the first time a rumbling/whining noise when stationary in park, reverse, and neutral. It was noisiest in reverse, but quiet in any of the forward gear positions.
First thing I did was check the oil level, and found no indication on the dipstick, so I topped it up, but the noise is still there.
Is there an easy cure, or does it sound like a rebuild? Any ideas, anyone?
Chris

If it makes noise when standing still, I'd check that the engine hasn't settled on the mounts during storage. Sounds like it's coming in contact with the body or frame somewhere. Check the exhaust pipes too. The sound might be coming from somewhere else. The engine shifting on the mounts will cause it to come and go in different gears. This kind of thing is common in vehicles with swapped engines/trans.