On my Miraphone 5050 I've been bothered by a bit noisy 1st valve spring. I'm sure it's the 1st valve spring because I removed the others in case they were vibrating in sympathy. Since I 'stretched it out some' last year, I was pretty certain it was not perfectly vertical. These are stainless steel and the heavier duty type (P/N: 003314400029) for the M5050. So I ordered some replacements from Badger State Repair. They came in, put in the new set and was hearing the same noise. It's not real loud, but I hear it and it's definitely spring noise. SO... I took my old spring from #1 and cleaned it real well in a small ultra-sonic cleaner I have then dipped each end of that spring into a can of Plasti-Dip. This is that stuff that's used to coat or cover the handles of pliers and such. For tools it takes maybe 10 to 15 coats to get it thick. I only dipped the spring 2 or 3 times. So far this seems to be working. It remains to be seen if this coating will come off like the plastic coating eventually comes off the Yamaha springs. Here's hoping. See pics below:
The bottom of the spring (larger diameter) I only dipped twice because I wanted it to still fit in the groove of the bottom valve cap seen below. The Miraphone folks are pretty ingenious designing their caps to accommodate two different sized springs. See below: