Jerry is right.
This is a rather rare effect involving only a few nanos.
The use of a ceramic resonator would surely contribute to
the problem. The best solution is to replace the nano (maybe with a real one?).
The new one would most probably not have the problem...Alternatively,
one could build one's own version with a UNO (or other arduino) and check out how that works.
In any event, it comes down really to replacing with a better unit, or shielding
the offending signal away. I suspect many problems would disappear with
good shielding and short shielded leads, (which incidentally is among the
recommendations of Farhan also).
One could spend a lot of time figuring out just where the RFI is coming from.
but to what end? The important thing is to have a rig which communicates.
Finding a proper VFO is easy enough...but to each his own; some are intrigued by
just what is happening and are not satisfied until they know all. Fine enough.