You purchased this horn at Dillon? I would contact Matt Walters at Dillon and have him check the horn for you. He is one of the best low-brass technicians in the USA.
FWIW, I have been playing a Sterling since 2009 and have had none of "buzzing" problems you describe. As regards "clacking" sound on 4th valve, your horn uses the same synthetic felts as a Besson Prestige/Sovereign. If your horn has cork and natural wool felts, you need to have them replaced. The buzzing you describe "could" be related to the trigger, but again, I would have Matt check it
Doug Ruby
Sterling Virtuoso 1065HGS & Adams E3 Prototype 0.70 Top Sprung valves
Sterling Virtuoso 1050HGS baritone
New England Brass Band
Winchendon Winds/Townsend Military Band
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