Tracking down a hum.

Today when I powered up my system, I noticed a hum from my tube pre amplifier, a Rogue Audio Model 99 (line stage only). It is worse when first powering up. It is absent through my headphones, (using an external headphone amp and the record button on the pre-amplifier)  Listening to speakers the noise is present but diminishes and eventually goes away as the system warms up.
I'm using the Rogue to send a signal to a Musical Fidelity X-P200.
I don't think the issue is the tubes as I'm using Tung-Sol 6sn7 tubes with less than 300 hours on them.
I bought this Rogue used and had to take it back to get it fixed as one channel kept cutting out because of a bad solder in the out put jack. Maybe something else failed?


Okay I got the amp back today. I was hoping for an itemized list of parts replaced. I didn't get one. It looks like the replaced all the capacitors in the board nearest the transformer, and one resistor was replaced. I'm listening now and so far so good.

Well after listening for a couple of hours out of nowhere the noise is back.Evidently the folks at Rogue missed something. This time the noise doesn't dissipate. The noise pulses. get louder then softer louder then softer. I have no idea what the cause is now. I'm thinking a faulty rectifier.
This is frustrating and starting to get expensive.


I had a hum that was caused by a plug in air freshener in another room.  When it cycled on I would get a hum or sometimes a weird buzz.   Thought it was my Amp, it was a tricky one to trace down.  

It would be something else in the room or also some DC on the line causing the transformer to hum.

@ jond I don't think its the line. I swapped out the amp with an old NAD C 340 using the preamp outputs and everything is fine, nice and quiet. No noise in any other components using the same outlet.