problem with high frequency noise

I have 2 projects, basically set up exactly alike. One of the two has a high frequency noise, between 16 and 20KHz, around -65dB, the other doesn't. 

I put an analyzer on the master bus, which clearly shows it: See here.

Same plugins on the projects, same busses, I couldn't find a difference that would explain this phenomenon.


Any ideas what might cause this? Any help would be highly appreciated, since the songs from this project are to be mastered this weekend - mastering studio got back to me telling me that 14 out of 19 tracks show this noise, the others don't. As said, they are basically setup completely alike.

Thanks,

when I disable all plugins, the noise is gone. Then I start adding plugins. With the first plugin, the noise appears (Stillwell 1973), with every other plugin the noise increases.
Although: as said, I use the exact same plugins on the other project, and there is no noise.

Other ideas?

Where can I change dithering settings?

One finding: in project A, I solo a stereo piano track. I insert plugin A, very slight noise appears. Insert plugin B, noise gets louder.

in project B, I also have a sterei piano track. I insert the same plugin A, no noise. Insert same plugin B, still no noise. Insert 5 more plugins, no noise.

SO: the same plugins introduce noise in project A, but don't do so in project B. This leads me to the question, which setting could be different that explains this behaviour ??