In some day i've noticed my car starts having an exhaust leak at the manifolds to downpipe gaskets which became shamedly loud in two weeks. As i knew it might be a manifold crack issue too it took a time for me to obtain non-cracked US manifolds. So in a one day we did reinstall it and found that downpipe gaskets were not even a bit similar to what it must be there. And my manifolds were not cracked (now i have spare hehe). So after we put everything back with OEM gaskets and nuts the car starts and idles as silent as it likely should and even more silent than it was when i've it. Also the idle became more smooth and the exhaust smell went away. But now until I've got my engine fully warm (like temp needle is just a bit off left from the center) i hear some unusual roaring sounds that definitely looks like exhaust not even a leak but more likely touches the body and it totally off (like somebody turned the switch off) when the temp is set as i said in the middle of the gauge. At the very end i also hear something that looks like slight metallic clicks in the rear end that is switching off too with the temp raise. It is strongly correlated with an engine temp. I have a street with 4 stop signs on it on my regular way out of home. when i got to the first i almost warmed fully but still have a sound. At second one i have the sound off but while i stop car cools a bit (i see needle drops slightly) so i roll off with a sound again which comes off in a couple of second as temp raises back. And all the way down next two stops car is silent and i barely hear it running as the engine temp is steady warm. If i take other way with no stop signs or lights i hear those sounds until the temp needle is vertical. If a leave car idling until fully warmed i hear no sound right from the drive away. And there are no sounds while steady cool or warm revving or idle. We've checked everything - took off the rear exhaust parts and kick and shake the cans well to check if something rattles inside. Nothing. Lifted the car and run it to check visually. Nothing. No noise. No leaks. No parts touching the body. Also there some roaring sound from the rear when i release the acc pedal while car is in the top gear (i mean torque converter is locked). Something that sounds like engine braking as i feel the car is definitely holding the gear (it did it before but looks like i didn't hear that noise because of downpipe leak). Some people said it probably a transmission as it won't get into the converter lock until hot and it comes like this with all those sounds. I doubt it as when i drive away home on the road with no stop signs or light the sound comes off right when engine is fully warm but i see the transmission is still cool as it won't get into the converter lock for another couple miles or so. For me it looks like the fuel injection does something thats causes exhaust to work louder until the temp is raised as it switching on and off like it's somehow controlled. Frustrated and sad. Car is 95 VDP 4.0 normally aspirated with like 78k on the dash.

 

When you hear the noise shift into N. Does the noise change?
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I feel the noise when it drives :-) If it stands still there is no noise idle or revving and no significant difference in vibrations or engine sound between D, N or P gear positions while standing. Just slightly lower at idle in D but still smooth. And forgot to mention. It is probably not the sound but a feel as friend told me he wont't hear anything extra from the exhaust when i drive away but i feel it from inside. Like it transfers through the car body i don't know. From outside it works at normal noise level. Might be really in a powertrain but can't realize how it is that strictly related to engine temperature.

 

How is the transmission mounted to the car? I am not as familiar with the x300, but the XJS has some Rube Goldberg type of assembly with springs and bushings that wear away. I would take a look at your transmission mount.

 

There are two vertical rubber mounts under the engine and the rear transmission isolator mount on a crossmember. This rear looks good but i recently obtain an OEM replacement which is highly likely will be replaced this or next weekend as far as trans fluid so everything underneath will be checked up twice more. Might be a cause BTW but usually bad trans mounts chime in early with the D position vibrations. My car is pretty similar smooth ar idle as i said already at any gear position. It just drives me crazy. Should this be a constant sound but it disappears with no traces until cold again.