The 3312A just has a simple VCO controlling it's frequency. Have a look at figure 7-5 in the manual, bottom or left hand side depending on which way you're looking at it. Either it's one of the power supplies drifting (17, 14 and -14), the VCO input, something coming in from the modulation generator or one of the half-dozen components that make up the VCO.
Let me know if there's any specific measurements you want made.