I had a noisy headphone output on my RM800 and was told to use cheaper, shittier headphones 🤣 I think it’s just an inherent part of the design, it picks up a tiny bit of the PWM signal of the SMPS is what Guy and Dave told me when I stood in their workshop. Don’t think it’s unique to your RM at all!
Anyhoo, great price for a head I really liked when I had it