First, are you SURE that the bearings are preloaded when installed????
Simple generic A/C's are not preload. ONLY if they are factory preloaded (usually designated by DUL, DUH or DUM in part number in NSK speak) can you be assured of a true preload. A random pair of 7207 bearings will NOT be preloaded. A factory matched pair of 7207DUL's should be preloaded and the preload is usually certified on a chart provided with the bearings. No cert chart, be wary.
Machined bronze cage bearings can/will rattle the cages, ESPECIALLY if you're using a VFD instead of a DC motor drive. The sinusoidal current of an AC motor rattles the heck out of bearings with metal cages. If it is TRULY cage rattle from VFD, then turn up the radio as this is not going away and it is a PITA but not usually terminal.
IF it is cage rattle due to shaft to housing misalignment, this can lead to cage breakage and it needs to be addressed/fixed.
IF the bearing has a plastic cage, they should be quiet as plastic cages typically won't/can't rattle. HOWEVER, if the housing is distorted and the balls are running into and out of a load zone due to uneven or no preload from distortion, you'll hear the balls howl or rattle about. THis will tear up a riveted cage and can pound out a plastic cage eventually
Regarding preload: generally speaking, low speed grunt spindles can use heavy (DUH) preloads. High speed spindles, conversely, use light (DUL) or ultra light preloads (designated by something totally different). For 10K use, I'd expect DUL.
FOr what should be obvious reasons by now, system squareness is even more critical if DUL and or high speeds are involved.
YOu can have the bearings checked for preload by KAF Mfg in Stamford Ct. They can inspect and reset the preload to whatever you want/need.
If this is an upper bearing, mounted adjacent to the drive belt, it might be that the belt tension is unloading the bearing.
You can NOT compare tapered roller preload to round ball bearing preload. TOTALLY different internal geometries. When you preload a taper, you're actually shoving the edges of the rollers up agains the shoulder of the cone. Thus, you have SLIDING friction. When you have a preloaded A/C ball bearing, you always and only have ROLLING friction. BIG difference that should now be self evident.
If the housing is distorted and the axis of the shaft is skewed with respct to the true C/L of the housing, you'll end up with the balls being unevenly preloaded or perhaps even unloaded for a portion of the ball path. They'll make noise as the balls go into and out of the load zone when this happens.
The spec for shaft and housing squareness needed for a high speed spindle often makes service and/or the machinists cry as it is almost perfectly square with NO room for slop. Reason: keep the balls evenly loaded ALL the time.
There is also a possibilty of grease noise. Some greases sound like popcorn in preloaded A/C's - it is the balls rolling over the thickening agent in the grease. Sometimes grease thickeners actually feel like marbles are in the bearings when you turn them.
HINT: wash out the bearings with Brakeclean and see if the roughness goes away when you spin it by hand (lube it with a few drops of DEXRON). DON'T blow gun spin the bearings or run them w/o lube. If they are quiet and free turning lightly oiled, use different grease and see if there is a difference.
To tighten the bearings under preload, assuming you have a slip fit, we use torque turn method. This loads the spindle nut to a torque and then so many turns more. The torque essentially takes up the slack, the turn sets the preload by compressing the A/C's to the point that the inner rings and outer rings touch - this is when the built in preload is finally established.
This spec is established by the OEM when they set up the spindle as it takes spindle nut torque, spindle stretch and/or bearing preload offset and preload into consideration. Your bearing supplier's application engineers should be able to help you with this spec as they can figure out what the ring offset is and how much you have to turn the nut after making contact to affect the proper preload if you give them enough info. If not, they are simply lame.
I after all this, something is probably still awry that you haven't found/fixed yet. You'd better find out what's going on before you even think of 10K. But by now, you now have more to go on....