Try a larger capacitor, smaller resistor?
I'd probably go with a 400VAC+ capacitor not necessarily X2 rated, because the X2 rating allows self-healing (the cap momentarily short-circuits), and after enough self-healing the value decreases dramatically. The voltage across a TRIAC can't be much over its rated voltage anyway, so there's no problem with transients. Anyway, it's probably a small difference, and not one I would expect to show up until after years of operation.
And yeah, capacitors buzz, the mechanism is electrostriction. Some do it more than others. Want to say polypropylene does less than polyester?
Tim