I have an old Kenmore canister vacuum model 116.2497582. While in use the motor intermittently changes tone to a higher pitch that results in at least a 50% suction loss. Sometimes it will correct itself in a couple seconds, other times percussive maintenance can be applied and that will stop the problem. The motor noise is higher pitch and different than the noise from suction being blocked. It sounds more like the motor has no load on it.

This vacuum is older than I am so the motor brushes were the first suspect, but they appear to be in okay shape. A little discoloration on one brush and the metal around it, but it still looks like they both make good contact. All the wires seem intact and the motor spins freely so I am at a loss to what could be happening here. Could someone please guide me in the right direction as to what might be my problem? It would be a shame if this exploratory surgery became an autopsy.

Dose the motor shaft rattle around in the bearings?

No, there is no play at all.

Any time the route through the vacuum is blocked it will reduce the load on the fan blades, due to its design, and it will spin faster. You need to disassemble the unit and clean it out. Make sure to clean the fan blades, too. Look for any filters that could be clogged, as well. I'm sure you will find the source of the problem. Be sure to oil the motor.

The problem can come and go in a split second so blocked suction has already been ruled out. The filter material in the exhaust area was very dirty along the edge but at least 75% was open. While I was in there, I oiled the motor and it seems to move better so that may have been the problem. I will report back once I can test it.

Vacuum cleaners have backward curve fans (a furnace uses a forward curve fan, in contrast) which build up very high pressure down from fan causing the motor to load up, whereas, if the restriction is toward hose end the motor unloads and speeds up. You can hear this when you cover the end of the hose