well, I can tell you it's not the tube!
I had a brand new renegade having the exact same issue! I change ALL the tube from spares I had! And still that annoying sound that appear after 30 minutes of warm up!
Since it was less than 30 days, I brought it back at GC in LA and got another one! No problem since last october!
Knowing it's not the tube, I suggest you contact egnater and benefits from that 3 years guarantee!
Good Luck!