[Elecraft] Hum on 15 Meters CW

[WD0ECO]

I'm hearing a vibrating humming noise from my K3 when keying CW on 15 meters.      

It is only on 15 meters and only with my beam antenna - not heard on the
dipole or vertical.  

SWR is 1.6 with K3 tuner bypassed (LP-100A meter) and 1.1 with tuner on and
according to K3 meter.  Tried different keys - still hum.  

The hum is not heard under 50 watts but gets progressively louder from 50 to
100 watts.    

I don't notice the hum on 15 meters SSB via mic.

This just started happening.  Any ideas?  It doesn't appear to affect output
or have high SWR.  Is it hurting anything???

73, Mitch

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