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Mine rattles a lot louder under load and over a larger RPM range when the engine is cold. For instance when I drive up the hill after I pull out of my driveway in the mornings, the rattling is very pronounced and I can literally hear it echoing off of the houses as I drive by but if I have to go out to get food after I get home from work and the engine is still warmed up I can barely hear it at all except for right at 1800-1900rpm for a brief second. Im sure the loose or broken heat shielding or whatever is vibrating will eventually begin rattling even in neutral/park. You may have to wait until the problem can be identified under safe conditions.

The maxima if I’m correct has 3 mounts and it ended up being the one on top of the engine. The bushings in it were toast. Sent from my iPhone using Tapatalk

I have removed a lot of my heat shielding because it was all falling apart and I do not notice any issues with heat damaging components under there. I too have rattling and I'm almost positive it is coming from resonance in the exhaust piping itself. I have observed the pipe vibrating when the sound appears. But I can produce the sound in neutral.. Have you tried power braking to see if you can find a spot where the rattle appears? If you can force the sound to appear while stationary maybe you could back up against something completely solid (if you have a tow hitch or something that wont get damaged), like a giant cement structure that you're 1000% sure cannot move, and then have someone apply power until the rattle appears and then you carefully find it with a long stick. The rattling should stop when you jam a stick against it.

The plastic splash guard attaches between the radiator support and the steering rack crossbeam. Check this and related parts https://sfcreation.com/product/r50-mid-skid-plate-1-4-aluminium-96-04/