Whichway,
Something you can try to tell if its bearings is to put a spot of heavier oil on the bearings, it will cut back your casting distance but if the noise is reduced or disappears you know what it is, also check the spool end play cap for correct ajustment, if the reel has seen a lot of work it could be also be wear in the level wind gearing or worm drive.
My Ultra Cast 4600 is quite noisy when casting, but this is because I use it to flick light lures about, so the bearings only have 1 spot of very light machine oil in each one, even so I would describe the noise as a busy whine not like it is about to explode.
Just check also that your new bearings are a snug fit on the spindles and in their houseings.
Hope the info helps
Howard