Other XC90 Problem: Vibration - Groaning - Grinding: Passenger Footwell (Solved)

[Sceptre63]

As with anything that ages, unfortunately that is everything, things go wrong and finding the route cause is sometimes not so simple.

I have been in a relentless spiral over the last many months working out the cause of a very deep grinding/groaning noise in my XC90. Relentless searching of the above words has driven me to check many causes from engine mounts, drive shafts, prop shafts, suspension, ball joints, steering racks, trims, etc...
and finally i found this last night with luckily only spending the cash worth a rear engine mount to solve my problem (However this was not the problem)! Ignore the hours as this is treated as a hobby (a slightly frustrating one to say!)

Symptoms:
-Vibration / groaning / grinding through the passenger footwell under load only
- RPM - 2200 - 2500 (irregardless of speed, so engine side issue)
-Create the noise while driving then hit into neutral noise goes away.

Fault Condition - Known issues:
- Rear lower engine mount shot _The trick to check the integrity of your engine mounts is to remove the oil filler cap, if there is a loud POP you have a vacuum leak.
- unbalanced tires on front
- rear suspension bushings wearing down
- developing steering rack fault

Suspected issue / description
- Rear engine mount - warn through the rubbers and metal on metal vibration
- Torque converter - fault in unit?, however the particular engine frequency driving the noise suggested this was unlikely
- AWD Transmission, assume one of the complex rings/ gear was shot/broken. Only place this could be is on the direct output from engine, all other outputs are gear ration dependent.
- Propeller shaft - assumed the damper/couplers was damaged and or the bearings were shot
- Transfer Box - assumed the unit was failing (i dont actually know what happens when a TB fails - do you just loose 4WD?)
- Top Engine mounts - assumed warn out (already after replacement circa 12 months ago)
- Drive shaft to TX CV joint. No abnormal movements observed.
- Driveshaft spline into the transmission - there is an small amount of movement - this is a developing problem, however this cannot be the issue as the noise is RPM dependent.

The corrections:
- Top engine mount BAR to Engine mount bolt the NUT was missing - This did NOT fix my problem
- Checked the prop-shaft no excess movements that would cause the issue. Note prop shaft is an OUTPUT of the TX and as such cannot be the issue as the noise is engine RPM related.
- Replaced the rear engine mount - this SHIFTED the grinding frequency to a 1800 - 2000 RPM.

The Solution:
Based on the above it was very clear to me that there was something else out there on the car that was the issue, suspension / mount related.

After a good drive to get the car war and all the rubbers warm also, i chocked the wheels, put the car in drive with parking break on and the wife in the drivers seat with the foot ready to hit the break (e.g. so there was simulated LOAD). I went around the car engine bay (NOT the FRONT!) looking / feeling / listening for the vibration. I lent on the passenger side wing and felt a subtle real deep vibration. This was not enough to make a audible noise similar to what experience at 2200RPM but enough to work out that is it something that is DIRECTLY bolted to the body / chassis.

It turned out to be the top engine brace. the Nut and Bolt through the bushing that eventually bolts onto the area near the passenger side suspension strut. The bolt was just loose enough to turn by gloved hand (with a bit of effort). Re torqued and the vibration is now gone.

I hope this experience helps others with a less serious issue such as a major failing transfer box, transmission, drive shaft or prop shaft and is a DIY activity!