Vacuum pump whining sound?

I have a Moroso 22641 four vane vacuum pump on my hemi. I bought it used, but everything looks great internally and seems to pull good vacuum. Took the car for a drive the other day and after a few minutes I noticed a high pitched whining sound coming from under the hood. Almost like a gear whine, but much higher. I started doing some checking and found it was the pump making the noise. What I did not check was to see if it still made the noise w/ the suction hose still connected to the motor.

The sound varies w/ RPM, but does not do it instantaneously. Leading me to think it is not a bearing issue. Almost like it's a sucking sound...maybe from a leak on the motor? If you rev it up, the rpm drops way faster than the sound tapers off if that makes sense. If it was a bearing going bad I'd think the noise would follow the rpm much closer.
I took the pump apart again and everything still looks fine. There was a small drop of oil on the bottom of the case... evidently a slight leak from the front cover to the case.

Anyone run into something similar?

I don't know how relative it is, but I have seen pcv valves get stuck open, and when the vacuum gets too high, it would cause the crank seals to suck air and howl like a reed valve. Since your noise isn't following rpm, I'm thinking some sort of gasket or seal might have a similar condition. Open your oil fill to relieve vacuum and see if the noise goes away.

I think that's exactly what's going on here Kenny. These pumps are very simple, pretty sure I'd have seen a problem w/ it. They way it sounds, it sure doesn't seem to be mechanical. I'll put the pump back together and get it back on the car and see what happens.
It didn't do this before I changed valve cover gaskets. I had em off to lash the valves and found one corner of a gasket sucked in. I'd noticed the vacuum had fallen off. New gaskets, vacuum is back up, now I have this annoying sound.

Sounds like a leak on the engine side, especially since it is responding to RPM but not following it exactly. If you have access to a smoke machine they can work wonders finding things like that. Also if it is sucking air they will usually be putting more oil in the puke tank.

Smoke probably won't work. Smoke pushes in and builds pressure, which may push a poor sealing area tighter together.
If it had a corner of the valve cover gasket sucked in already, the problem might have been there for a while, but didn't show its face due to low vacuum. Higher vacuum is forcing the issue.

Smoke works great everytime we have used it to find a leak. The smoke contains a phosphorescent dye in it so even if you do not see any smoke with a black light you can see where a leak was. There is zero pressure behind a smoke machine. We use them all the time to find leaks in Evap systems and even on the off chance it sealed a leak the dye would tell the story.

my moroso made some weird noises when the belt was too tight.

Just an update...it's definitely the pump itself making the sound and it still did it even w/ the hose disconnected from the valve cover. Took the pump apart and all looks good. I loosened the belt a little and it seems to have gone away. Still seems to be working fine.