Amp troubles (reverb issue)

[IFoughtTheLaw]

So I have had a Marshall MG15CDR amp for a little over 4 years now ( I got it when I was just starting out) and just recently I noticed that something is going on with the "reverb" on it. I usually play it with the Bass and Treble knobs at 10, and Contour knob at half. When I use the Reverb, I just turn the knob up to 10. It usually has sounded pretty good, but now I've noticed a sort of "buzzing" or "crackling" that happens at the last second or two of a note when I have the Reverb up at any level. If I strum a long note, as the sound is fading out, it crackles, and if I am just playing a bunch of short notes or just picking, it crackles at the end of almost every note I play. It does it no matter what settings I have the bass, contour, and treble knobs at.

I have those "energy saving" lightbulbs in my room, but I have had them for over a year now and they have never caused a problem. It does the same sound no matter what cord I use, and no matter what guitar I use. My dad thinks its the speaker that is wrong, but wouldn't everything sound off if that was the case? Plus, I hear the crackling even when I have emulated headphones plugged into the amp.

Keep in mind that my dad most likely won't let me open up the amp to do any sort of work or adjusting to it. We were thinking of just bringing it into Guitar Center to see what they can do. Any thoughts as to what may be going on? I can maybe try to record the sound the best I can from my digital camera sometime tomorrow if you guys don't understand what I mean.

[øøøøøøø]

Is the reverb tube driven?

If so, I'd start there.

After that, I'd be looking at the reverb tank.