It could be a lot of things. Try pulling tubes one by one and see when the hiss vanishes. When you pull the detector, all thats left is the audio, and if the hiss remains, you know where its coming from. Cheers de Gord, VE3EOS -- Gordon L. Hayward, Ph.D., P.Eng., Associate Professor, Biological Engineering, School of Engineering, University of Guelph, Guelph, Ontaro, N1G 2W1.