Oil Leaking   
Probable: 1. failing seals, 2. mating surfaces
Remedy: 1. replace seals, 2. rebuild gearbox

Gearbox Running Hot
Probable: 1. Incorrect oil level, 2. inadequate air flow, 3. excessive rpm or load, 4. contaminated oil, 5. failing bearing.
Remedy: 1. check oil level, 2. provide additional cooling, 3. change to synthetic oil, 4. replace oil, 5.replace bearing.

Unusual Noise
Probable: 1. gear mesh changed, 2. excessive external load, 3. failing bearings or gears.
Remedy: 1. inspect drive line, 2. inspect belt tension, 3. replace bearings or gears.

Vibration  
Probable: 1. loose mounting bolts, 2. loose couplers, 3. pulleys, 4. failing bearings and gears, 5. drive line misalignment. 
Remedy: 1. inspect/tighten, 2. inspect/tighten, 3. replace bearings or gears, 4. correct misalignment.