Probably is too much gain. First, all tubes are microphonic to some extent, especially in a high gain input stage. Yes, some a lot more than others, but newness is no guarantee of zero microphonicity.
meanwhile if the guitar is turned all teh way up for this, stand back as far from the speakers as possible. Does it lessen? Also, whatever way you are facing with the guitar, try turning aside, like aim the guitar 90 degrees from what it is now. I am asking if the direction the guitar faces has any effect on this symptom.
You can have pickups feeding back in high gain circuits even at low loudness.