Well, to eliminate NOISE I guess you would need to soundproof whole vehicle, which is quite a huge project. Maybe some quick tips I did on mine:
- custom trim polstering on back I made out of an old 3/4 inch thick dampening trim from Ford Mondeo
- I bought some sound dampening self-adhesive sheets 10X10 inches, which were quite expensive, cut them into 2 or 3 smaller pieces and put them on larger metal areas all around the vehicle. For example, inside doors there are some metal plates, which vibrate, then put some under pedals on bare metal, and some in back on wheel wells metal etc.
Transmission bypass...well, this is one of the most popular topics on Pathfinder community, so there's no ultimate guide. Most of guides are good, and AFAYK there are two main approaches:
- do a plain bypass
- or go with aftermarket cooler and do a pro bypass with cooler
You could add an aftermarket transmission fluid TEMP gauge with additional temperature reading sensor, or buy Bully Dog tunning device, which can read tranny temp out from vehicle LAN system.
Oil change? Use some good oil, for example I use Shell Helix Ultra 5W-40 (some 50 bucks) and most expensive Oil filter (some 20 bucks).
As I did not know the vehicle mileage from the last oil change after I bought it second hand, I did an oil FLUSH with 50/50 mixture of cheap synthetic motor oil and diesel, then again one FLUSH with pure cheap synth motor oil, and finally put Shell Helix Ultra 5W-40 in. But those flushings are my own preference, so I cannot recommend to anyone else.
Anyways, inspect OIL PLUG magnet for any significant metal particles. Some dust/debris on magnet is OK, while metal parts should trigger alarm.
I'd also check AUTO tranny ATF fluid level (do it while engine running idle after at least 20 minute trip). If level is OK, turn OFF vehicle and with a tiny hose and huge injection (you can get one for few cents in drug store) pull out some ATF liquid via level checking tube and inspect ATF wear and smell. If it smells BURNT it's not good! Also if it is pitch black it's not fine, either. Both symptoms call to immediate ATF change or even flush + change.
But do NOT just add ATF if level is low, as you cannot be sure what's inside.
I'd also change all diffs oils and central transmission box ATF, but as cheap those changes are, they are pain in the ass to do it without oil pump and in your own garage.
Maybe lift it up and inspect propeller shafts and drive shafts for any play, check brake pads (YES, they can fall off if worn beyond limits!), check front driveline for any play.
While lifted, check also inner tire walls for possible bulges.
Check other liquids, coolant, brake, steering and also windshield washing fluid.
My 5 cents
