Hello!  I have a Marshall DSL40c, which I absolutely love.  I swapped the speaker out for a Celestion Vintage 30 (16 ohm) a little over a year ago, and it sounds great.  However, I recently got a lower wattage tube amp (Vox AC4TV) and have been running it through the DSL40c (basically using the DSL as a speaker cab, with the power chord disconnected). 

So here's the problem:  After 2 days of using this configuration, the DSL has started making a sound when I play the vox through its speaker.  The best way I can describe the sound is that it is like a tabmourine, like those little metal disks jingling together.  It does this on certain notes, like the open low E string, or a C chord, but not on other notes like a B chord (i.e. 1 fret away).

When I run the the Vox alone through its internal speaker - no issues at all.  When I run the DSL amp section alone through it's vintage 30, no tambourine sounds, although it seems that the green clean channel is lower volume than it used to be (the other 3 channels seem to me the same volume as normal).  There is also some instability in the volume (goes in and out, but in a sublte way).  There is some clicking and popping when I first fire it up, but this stops after it has been on for a while so not sure if this is normal or not.

I have been looking up reasons why amps may rattle, but everything I am finding describes issues with the tubes that are being played through - my issue seems to be a bit different, since the rattling (tambourine jingle) sound I am hearing seems to be coming from the Marshall when I am not using its tubes (playing through the Vox as a head).

Is it possible that a tube has gone bad in the Marshall, and when I play the Vox through the Marshall's speaker, the vibration is rattling the broken tube and making this tambourine sound?  Would a reduction in the volume of the green clean channel imply it is a preamp tube vs power amp tube (since the other channels are unaffected?)

Thanks for any help you can offer.  I don't mind taking it in to a tech, but I have read a lot of posts of people who took their amps to techs and had tubes changed, etc. and the problem persisted.  I want to have a pretty good idea of the problem before I start investing in having it fixed.

Thanks!