RE: Low grade rumble at idle when center diff locked

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> Can someone explain to me where this rumble sound / vibration is coming
> from? What effect should a locked differential have on an idling engine in
> neutral? There is certainly something I am not understanding here. Can
> anyone help? - 86 4KCSQ

I've experienced that on my ur-q and a bit on the '85 4kSQ.  Check your
engine, tranny and rear diff mounts.  Since clutches last so long in these
cars, tranny mounts are usually still original.  When I did the clutch on my
'85 (@ 206K miles) there wasn't much left of the tranny mounts.  Remember
that when you lock up differentials you can actually build a static torque
between wheels and drivetrain.  This can end up causing driveline parts to
move relative to the body of the car ... even with the car idling in neutral
...

HTH!
Steve Buchholz
San Jose, CA (USA)