Speaker pops when AV receiver turned on

[ScoobyDoo]

I've got a yamaha rx 675 av receiver and 5.1 surround speakers.
I've had this for approx 6yrs.
For the past few days when I turn on the yamaha I get a loud pop through one speaker. If I leave the yamaha on for say 5mins, turn it off and then back on, I don't get a pop.. Its only when the yamaha has been turned off a while.
Nothing else in my setup has changed.
I've tried changing the speakers around and unplugging one or two but I still get a pop through a different speaker.
Any ideas?. Is the yamaha on its way out?
Could I leave it tuned on constantly so I don't need to power it down?

 

[ScoobyDoo]

Thanks for the reply.
Yes the amp is in an enclosed cabinet but ive taken the rear of the cabinet out to give it some airflow.
Theres nothing sitting ontop of it blocking the air vents. Its been like this for the past 7yrs.
Looks like it may be failing like you say and its time for a new receiver.