Well, the whine is back, and so is the foaming. Since I have a new reservoir on there, I reached underneath the inlet hose that goes from the reservoir to the pump and found a small fluid leak. I suspect that's where the air is getting in.
I put some White Lightning grease on there, but thought maybe something like Flex-Seal is better. My preference would be to replace the hose, but per the service manual, ID appears to be 11/16'', and both my local parts store and (surprisingly) RockAuto came up empty on the part.
Any suggestions? I do think this is where the air is getting in -- there are no other apparent leaks anywhere in the system.
EDIT: I think I may have found the part on the Sunsong site, but description only--no specs or picture. Have left them an email.
(I'm also in contact with LucasOil about fluid compatibility--thanks for that tip)