My 2002 Lexus SUV RX300 has about 88k and has been fine, but is now making a grating sound when accelerating from a stop (e.g. from a red light or stop sign or nyc traffic). It still drives fine but the sound is disturbing and concerning. As if a flap or metal shield or such is scrapping against a tire (nothing is visible) but only once the car is warm. What could this be???
Its most likely a flap or a metal shield, as you say, but just rattling against something other than a tire. Its probably a heat shield on the exhaust system as this is a very common thing.
There isn’t any way to say much for sure because there are a large # of possibilities. Someone is just going to have to get it on lift and take a look.
I believe that cigroller is correct.
Nowadays, most passenger vehicles have several heat shields, placed on various parts of the exhaust system in order to shield adjacent components from the very high heat of the manifold, catalytic converter, or exhaust pipe. As a vehicle ages, it is very possible for road debris to bend or to loosen these heat shields. Because they are just thin pieces of sheet metal, a heat shield that is not in its exact original position may make some very annoying noises, and this tendency can change with ambient temperature and with engine RPMs.
As was said, you need to have the vehicle put on a lift in an effort to duplicate the noise while a mechanic is observing and listening from underneath the car.
As mentioned, the first step is to get it hot enough to make the noise and have someone inspect it.
Just in case no one is able to pinpoint the source, I will relate a problem like this I chased down one time. Only made noise when hot. Rattling sound on acceleration/deceleration. Sounded exactly like a loose heat shield. Would not make the noise if it was hot and I shook the exhaust system by hand (even violently). It was the catalytic converter disintegrating internally.