Mine makes a weird rattling noise when fully hot (after 20 miles) when at idle in gear. It makes it since day 1, and it's not heard inside the car, but it sounds "unlexuslike" outside. It's definitely from one of the accessories, or the idler pulley. This only happens at the ridiculously low idle rpms, but as soon as you rev the engine to even 800 rpm, it's gone.
Just curious if all cars are like that.

Thanks gentlemen.

Mine does that too! Same situation you described, but mine sounds like its coming from the transmission area! Put it in park, or rev it up, and it quits... Keep forgeting to bring this up when I have it serviced.

Could be one of the exhaust heat shields. On my car I had to bend on of the CC heat shields for that noise to go away. It also was making the rattling noise only when at idle when hot.

Chris K.
95 SC400

Thanks guys. I'm sure the noise is coming from the front, but I'm not sure where.
I'll put my automotive stethoscope to work one of these days. I'm almost sure it's coming from the idler pulley. AC on or off doesn't make a difference, and in neutral the sound almost disappears.