Caveat - I don't think the suggested cure will work, depending upon the frequency of the whistle.
I've never even seen Cubase but the principle is simple.
You need to get the (parametric) EQ set up on the offending track. Usually a mid band. Set the Q value to as narrow a curve as you can. Set the gain to as high (boost) a value as you can. Your curve should now have a large narrow hump above the line.
Starting at the lowest frequency you can reach play the track with the EQ on and slowly sweep the frequency up. The track will sound pretty awful - that's the point - at some stage, that whistling is going to really jump out and hit you.
Stop.
You've identified the annoying frequency. Now reduce the gain at that frequency to cut as much of that frequency out as you think it needs. It's called a notch filter.
Better idea is to work out the cause of the whilstling, stop it, and re-record.