Tricky. I take it this is something you can’t remove with a notch EQ?
If you can’t get the bass player back in for another take (best option), SpectraLayers or Izotope RX would be your friend here. SpectraLayers has a free 30 day demo
but if you’re not experienced with spectral editing it can take a while to get decent results, especially for something like this where the rattle will presumably be most intense at precisely the same time as the bass notes are sounding. If the wanted and unwanted audio share similar frequency bands, you might need to zoom in until you can see each of the individual harmonics making up each note, and then chop in between them to take out the unwanted rattly stuff.
Having said that, once I got the hang of spectral editing, I now don’t know how I ever managed without it!
Sticking to what you can do within Cubase, I’ve sometimes managed to rebuild dodgy bass lines by creating a MIDI copy of the part with Variaudio, and then using a software bass synth or sampled bass instrument to play the midi part. It depends a lot on the material as to whether this will work or not.The MIDI as generated from Variaudio almost always needs some tweaking and cleaning up to play nicely on the replacement instrument.
If as you say the rattle is really only confined mostly to the higher frequencies, you could perhaps duplicate the part and then use frequency bracketing so the lows+ mids are coming undisturbed from the orig. part and the top end from a new part reconstructed as described above. Just be careful to check phase alignment. If you’re not familiar with it, the general concept of frequency bracketing using Cubase is described pretty well here:
If it’s just a few notes here and there that rattle you could search the part for clean notes and cut/paste them over the rattly ones. if you can’t find a clean note of the right pitch/duration you could again use Variaudio to “build” a note and paste it in.