Do you use a DI box in the signal chain? I had the same problem and solved with a simple Behringer DI box which eliminated the hiss.
Timely advice for me also. My partner's plug-in uke has been hissing. I bought a DI box (passive) years ago at the advice of a guitar salesman, and have never used it - I'll try it on his uke tomorrow.
What exactly does a DI box do, anyway?