Don't worry about it. My car has made that noise since I bought it. I did about 13k miles as a TT, then back in March I converted the car to single, it has not got worse.
I removed the aux belt and ran the car, the noise is still present. I changed many parts and it still remains;
Cams, cam pulleys, cam belt, cam idler and hydraulic tensioner, flywheel, turbos swapped for single, engine mounts, coil packs, crank pulley... you name it, it still makes the noise, compression is good and the engine is healthy making ~700hp.
Not all supra's do it, I had an Aristo that made the same noise, but not quite as loud. I've also just worked on another supra, and I noticed it made the same noise.
It's a harmonic noise, I'm not sure what causes it.... Casting flash in the head maybe, slightly out of shape cam journals? But it doesn't seem to mean you have problems. If I ever eliminate/solve the noise I'll post it on here so the world knows.