I just tried my hand at some loco surgery. I've narrowed it down to three Atlas GP35s which are "whizzzers" (I have two other used eBay Atlas GP35s which run/sound great!). I made a claim on two of the loud GP35s with the sellers, so we'll see how that turns out. The third one, I attempted to repair . . .
So, I pulled this "experimental" loco apart (it was the cheapest one), and on initial inspection, the trucks and internals appeared very clean with no visible debris. The bearing blocks appeared properly seated, and the trucks were oriented correctly. I then sprayed the internals with what I thought was a mild electronics cleaner (it contains trichloroethylene, which apparently is not safe for plastics). I'm going to go back in and try to "wash-out" any trichloroethylene residue with some isopropyl alcohol later.
Next, I put a single, very small drop of Labelle 108 light-oil on each worm-shaft assembly. I put it all back together, and the "whizzing" seemed to lessen by perhaps as much as 50%, but it's still louder than my other "quiet" locos. It did also, however, stop "screeching." When re-assembling the loco, I tightened the chassis nuts, possibly tighter than they were, so that may have helped to lessen the noise.
Now, I just re-read the "bearing block magic" article (thanks, Jeff!), and now I know what I'm looking at. I'm going to pull the loco apart again and give that a try. Then, I'll try to get a tiny drop of oil into the motor spindle (which I didn't do before). Next thing on my list is to get a proper set of tools for re-assembling these tiny things!