Impact noise from gears that carry no load, such as idling gears in a manual transmission or unloaded stages, hammering between flanks as engine firing pulses or torsional vibration jerk them through their backlash. It is worst at idle in neutral and in diesel drivelines. Reduced backlash, dual-mass flywheels, drag on loose gears and thicker oil all reduce it.
Rattle's only there at hot idle in neutral and quits by 1,200 rpm, that's gear rattle off the firing pulses, not a bearing.
Also heard as
- gear rattle
- neutral rattle
- idle rattle
Do not mix it up with
- Whine: Whine is a steady tone from loaded gears; rattle is random knocking from unloaded ones.
Related terms
Backlash
The play between mating teeth: how far one gear can rock before its teeth contact the other gear's teeth.
Whine
A steady, pitch-changing tonal noise from gears, rising with speed and caused by tooth meshing errors.
Clunk
A single heavy knock when a drive takes up its backlash on a load reversal, such as on and off the throttle.
Crash box
Shop name for an unsynchronized manual gearbox, where shifts grind unless engine and road speed are matched.