when starting the engine i hear a rumble/grinding noise. It is only from inside the car. nothing in the engine bay, and i can not hear it underneath the car. sounds like it comes from behind the dash, or the lower part of footwell.. disappers after about 30 seconds usually

 

Cabin blowers , fuse # 11 / 20 amp . LH heelboard and # 11 / 20 amp RH heelboard

A / C trim motors

Turn off steering column tilt and retract rotary switch

 

😀
i did swith off fans and ac but it was still there. will there be any difference if i pull fuses? i will try to switch off column tilt..

i know steering tilt only works in and out. up and down is inop.

 

One of the column directions has a flex shaft that shears and the 1 of the 2 motors never reaches the commanded position

Fuse # 13 / 15 amp LH heelboard fuse box

 

Could be the harmonic balancer starting to de-laminate. Mark the pulley across the grooves and the hub at the innermost (alternator) belt. Check the alignment of the marks after you run the engine.

Maybe you've done the balancer on the six already? The twelve is similar. Same size large bolt.

 

Wouldnt i have heard that in the engine bay?
no sound when i open the bonnet.. or under the car..

 

Possibly but not necessarily. I could hear a noise like that from the cabin and the engine bay with my old VDP six, but its balancer was really loose. I think the vibration could possibly be transmitted to the cabin without being heard outside of it. Like the XJ12 fuel pipe rattle, or the noise from the incorrect transmission mount repair in my car.

How is the charging voltage on the gauge during the noise? If it is unsteady or low, then it is likely the balancer. If it goes back to normal after the noise quits, there's another clue.

The balancer on my XJ12 was screeching at start-up, so it was pretty easy to tell what was happening. But they don't always screech.

 

Steering column motor trying to engage.

 

Today I disconnected blower fuses and tilt motor.

still the same unfortunately.

opened the bonnet and tried to listen to the harmonic balancer but its quiet.
And the voltage needle stays put in same position without movement.

but now i am starting to wonder if it is related to the exhaust. jumped in the backseat and the noise was louder there. popped open the baggage compartment and could even hear it in there when i stuck my head in.
but i cannot hear anything at all from outside the car.. tried to lay on the ground with my head as far under the rear section as possible, but nothing.

i think i will have to get it up on a lift to have a look underneath. it disappeared again if the rpm were raised a bit..


thanks to everyone for suggestions so far!

 

Fuel pump ?

Mounting foam surrounding pump ?

Turn the key to on position only and not start position

The pump will only run for 4 seconds by design

The pump comes back on with engine rotation

You have 2 pumps on the V12

 

interresting .. they are located under seats or above the tank?

 

above tank on the inside of tank

 

Emissions AIR pump? The pulley clutch is on a timer for warm up after engine start. That would explain the 30 second timing. Perhaps its a failing clutch and the noise is transmitting through the body and dash?

You can put a long screwdriver on the body of the air pump and listen through the handle. Or a mechanic's stethoscope if you have one.

It should be noted too that I had an intermittent whine in the interior of my XJ12 that changed in pitch with engine RPM. Went away with the engine off. I looked all around under the hood (and the AIR pump) and found it went away completely when the radio was powered off. I had an aftermarket radio, installed well using adapter harnesses and such, but even with the volume at zero, this whining noise (probably feedback from the injectors) was transmitted through the speakers. I was so sure it was a failing pulley under the hood. Car noises can be hard to track down sometimes.

-Nick