You can walk a tri depending on the distance. Admittedly probably not an IM distance unless you race-walk faster than some runners (and some race walkers do).

I'm planning a half-mary in March and I don't run at all.

Disclosure: no medical training or experience whatsoever but I do have horrid knees, really bad knees and joints. You might have something kinda floaty bits floating around in there, sometimes lodging where it causes a noise.

You could have arthritis and what you are hearing is bone on bone. Does it hurt?

IM very HO I don't think ultrasound will do much other than make it feel all nice and warm and make your chiro's reimbursement for treatment feel all nice and warm. OTOH if the ultrasound makes you feel better to the extent that you can do the work to strengthen the joint then it's all good.

Glucosamine may help, so would strengthening and stretching the muscles around the knee. Take the stairs, do some hill work, see a PT not necessarily in that order.

Actually .... in reverse order.