Help to isolate noise

[hookoo3]

I need help to identify a noise which is getting louder. At first I thought the noise might be tire wear however my fronts are new and the rears were replaced about 5000 km ago. One tire dealer thought it may be cupping noise however the noise is now getting louder with a more metal grinding type of noise. I've had the car up on jack stands and do not hear the noise when engaged in 1st. I do not think it is the bearings as I don't hear any noise when on the jack stands or when driving and holding out my head out the window. I've also now replaced the front motor mount which was worn out and the metal grinding low pitched noise is still there. The noise can be heard in all gears and with the clutch pushed in or out. It does appear to be on a constant rotational basis. The noise seems to come more from the center of the car and is now a metal type of grinding noise on every rotation. Any suggestions on what next to check out is appreciated. Thanks.

2001 Boxster S 6sp 93000km