Faint speaker hiss

Was wondering if anyone here might be able to help me out. I recently acquired a new naim integrated (XS2) to pair with my Harbeth p3esr. I am using straightwire speaker cables which I've had for years. I noticed when I plugged everything in that the speakers have the ever so slightest hiss coming from them. I cannot hear it unless I get extremely close to to tweeter and te sound becomes non-existent once music starts playing. I have had the amp for about a week now and have kept it on 24/7. Does anyone know where the small hiss could be coming from? I tried different speaker cables/power cord and that didn't change anything. I also tried unplugging all my sources and left just the amp and speakers plugged in ad still had the faint hiss. Like I said, it's not audible unless I really get my ear close to the tweeter (about 3 inches). I might be nit picking but was wondering if anyone has had experience like this before? Should I try a different outlet? I am in an apartment so only have one or two places to put everything. Any help would be greatly appreciated. Thanks!