Slight grinding on the PTO?

While out brush hogging, only thing I have that works the PTO, once and awhile when the blades go to spin back up the pto will make what sounds like a light grinding noise. Different sounding then the transmissions gears but I would definitely say grinding.

It doesn't do this with the finish mower. The clutch on the brush hog is new and is slipping when this happens on the tractor. The blades won't be spinning at all but the pto shaft is spinning, and once that blades start to spin up it happens.

Ideas?

What tractor are you using? Can you lift the mower deck up before starting the blades to make sure you are not catching a bunch of brush in the blades at take off?
On bush hogs, the blades rely on centrifigal force to straighten out and cut. This is so they my "fold" out of the way if they hit something solid on the ground--at rest, the blades often "fold" inward--when you first start the mower, they will jump and shake like the dickens until they are straigh out and "humming". could this initial jumping what you are feeling?
Lastly, maybe your clutch is TOO loose?
I"ll be interested in hearing others comments.
good luck, mark

brain fart on that one, TO-35

I wouldn't say it is the blades straightening out, I know that feeling and them being jammed up as I mowed for a few years at the Road Comiss.

It seems to be more if the blades get slowed down or stopped and the PTO has to bring them up to speed. This is even in heavy/thick field grass that it will do it, not just brush

If it only makes this noise on the Bush Hog and not the Finish mower then you can rule that one out. You say that the Hog is not turning when this happens but the PTO shaft is, that would seem to indicate that you have a tooth or teeth missing in the Hog Gearbox which is odd as the Gearbox is really Heavy Duty and usually you would hear some kind of noise all the time it was running. But if the PTO turns and the Hog doesn't, not much else it could be. BUT if it does happen as you say, just as the Blades start to rotate, it could also be just the load on the Gearbox because you say it stops as soon as the blades reach their proper speed. Have you checked the oil in the Box.
You don't say what Tractor you have, but are you sure the noise is from the Hog, it could sound as though it is,as it could travel down the PTO shaft, but it could also be from the Tractor. If you have a TO20 or 30 or a TE20 it could be the Bearing on the PTO shaft, if you have a 35 of any kind it could also be the PTO shaft Bearing and it could also be the needle bearing in the front end of the Hydraulic Pump drive shaft, as that drives the PTO. They can and do collapse after a long period of time and it may only make some noise when there is a heavy load, like as it starts up the Hog.The Clutch on the Hog should not be slipping like that when it is starting up, have you checked that nothing is sticking and keeping the two parts of the Clutch from engaging fully, have you greased it. If it slips like that it will wear away. When you spin the mower up, you should not have more than 1/3 throttle or it will snatch at the drive train and that could cause the noise too and the Clutch to slip..John

Is there any way to check the PTO bearings without major disassembly?

I don't think it is the brush hog as it acts normal the rest of the time. I check the fluid on every use as the bottom seal is weeping.

John when I say just as the blades start to rotate, I mean spin back up from being hung up on something, not under normal PTO spin up.