Diagnosis
This is a sign of excessive friction in the gearbox. Likely causes are old, dried-out grease or a misaligned gear causing binding.
How to Fix It
The gear case needs regreasing. Disassemble the gear head as described for bearing replacement. Clean out all old, blackened grease with a solvent. Apply a fresh, high-quality moly grease specifically for angle grinder gears. Do not overpack it.
If That Didn't Work
The motor armature might be rubbing on the field coils due to a bent shaft or failed motor bearing. This creates massive heat that transfers forward. This requires motor replacement.
Heads Up
That heat is cooking your seals and bearings. Continuing to run it hot will lead to a complete gearbox seizure. Let it cool down fully before investigating.