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?