Sample white noise and then use a non-realtime one in Soundforge 7 ... probably more accurate.

It's actually even easier to spot your room modes by playing notes on a keyboard and making a note of which ones sound funny in different spots in the room then converting the notes to pitch.

I've spent ages looking into this room problem and whilst I can tell you all it's faults I really can't work out what to do about them without spending a fortune ... anyone who thinks they can fix it is welcome to come around and give it a go and i'll make as much tea and coffee as necessary ! :)