Noise come from thre varactor

I just designed a low phase noise VCO for the X band. I found out that the varactor "inject" noise to the VCO in the midle tuning range. When we replaced the varactor by resistor and capacitor the VCO performance is prety good.
Do someone has experiance with this essue? Do someone has theory material related to the noise come from the varactor?

D.J

I don't have it available now, but I remember U. Rhodes book on PLL synthesizers had some good explanation on varactor related noise.