Buzzing from speakers. I have a sony str db1080 Reciever amp its about 5 yrs old. It has started

Question

I have a sony str db1080 Reciever amp its about 5 yrs old. It has started making a buzzing from teh speaker channels. I have tidied up all wires and have tried switcing off all potential interference sources like TV Sat box etc. but it makes no difference. The sound gets louder when the volume is increased. I have noticed that the longer the amp stays on the sound diminishes.
Can anyone help?