New hum after changing strings?

I have a Lakland JO5 with Lindy's which had some unknow strings on her. I recently broke the B string so I bought some new Roto swing 66's 45-130. Since I put them on I get a hum when I am not touching the bass. It must be a grounding issue, but can changing strings do that. I think the old strings were Labella round steels. The only difference I can see is that the old Labella wraps did not reach the string keeper above the nut. These new Roto's wraps go below the keeper but still above the nut. Any ideas. Please