Moocher said...
Have a 15m masthead rig with swept back double spreaders and baby stay. Sitting on anchor in a fresh breeze 18-20knots the mast pumps probably about maybe 15-20mm non stop. Mast has a sweet prebend curve, forestay probably not as tight as it should be, caps have no slack in 25 - 35knts hard on the wind, wondering if the rig isn't too tight. Any advice would be welcome cause it's driving me nuts and not sure what eventual damage can happe, thanks in advance.

Probably best to talk with your rigger.

Just pull the back stay on!

Most rigging failures are caused while at anchor or on a mooring. The shrouds will seem very tight at anchor and I would think swept back spreaders would add forestay tension hard on the wind like a dinghy rig. I would add more backstay tension and see what happens. The Selden rigging PDFs would probably provide good advice.

After tightening the back stay, it seems the mast pumping which is visual and rattles the halyards internally is still happening. The vibration now seems to be happening around the second set of spreaders and above, in fact right now is going off in 20knts. I'm wondering if I tighten the the forestay will this tighten the D2's which could stop this. In fact now since tightening the back stay it's even worse it's ridiculous. I will speak to a rigger, but having been around a bit I'm a little gobsmacked by this??

Jolene may be on to something. The old von Karman vortex shedding is an engineering classic. Google " Galloping Gertie" and Tacoma Narrows bridge.
My guess is that you have a furler, with the furler halliard taking tension away from the forestay. The furled sail can then excite a vibration in the system when it starts to shed its vortices at the resonant frequency of the system.
It is also possible that the mast itself is the culprit.
There would then be only two possibilities: Get rid of the eddy shedding device (furled sail), or change the resonant frequency. The latter is probably preferable, and you would need a smart rigger to adjust the rig tensions.