Clicking noise when adjusting volume

Hi there everyone. I've got a question about a funny noise my receiver started making a month or so ago.

It's a Yamaha RX-V2300 purchased in 2002. Whenever I adjust the volume, a slight clicking noise is produced in my speakers with every dB increment. Its not a loud clicking by any means but it's enough to notice during softer songs and parts of movie soundtracks.

This noise occurs with both analogue and optical audio connections.

Any ideas? Is this a receiver problem? Speaker problem? I've already tried different disc players and it's making the noise regardless of the source.

Thanks very much in advance for any help!