Buzz or hiss during some but not all vocal recordings

[Forenzikks]

Ok I got an OktavaMk 319 running to a Presonus tube pre amp and into a 2496 Audiophile and lately it seems that every other recording has this hiss or buzz..it s not every time but only sometimes.i ve checked wires and unplugged them and replugged them back in and it doesnt seem to be a certain one thing thats causing this.....All cables and equipment is brand new except for the Microphone XLR cables which i am borrowing so Im wandering what could cuz this hissing.....

By the way the hiss is only when someone speaks into the mic the rest of the recording is just a flat response with no hiss.

[Cliff K]

Just a thought: moisture on the capsule from singing? That can cause the mic to crackle.