[AlCan]

Could be many things... Lower (rear) engine mount? Exhaust system? Muffler? Trim... Not really enough information. What year is it? When you say "underload", what do you mean? If foot on brake, I'd expect No Load.

Anyway, my suggestion is try to find a safe spot, like your driveway, where you can set up the car to make the rattle, then get someone else to wander about the car trying to pinpoint the general direction of the sound, and go from there. Of course, keep your foot on the brake pedal! Or swap places...

I'm presuming it's a PowerShift, so letting your foot off the brake will cause the clutch to start engaging. You don't want to prolong that, not to mention being potentially dangerous.

All cars end up becoming money pits if you wait long enough... If it is a Powershift, read up here about changing the Primary Transmission Filter, if you want to keep it past 200,000-odd km.