Noise    Database

What Type        Where Is originating From        Inate Noise

Hiss
Crackling
Pops
Popping sounds can be caused by a poor connection between the speakers, speaker cables, and amplifier. ... Wires can also become pinched in furniture or damaged in other ways, which can cause them to fray and make popping sounds. Check the cable at the amplifier and speakers and replace it if it is damaged.
Turn off any nearby electronic devices that may be interfering with the speaker sound. Move the speaker wires away from any electrical cords.
Pioneer Crackling

Tansistor?.l,kjhgda
Corrosion on pin trafics into the transistor.

Take every control and exercise it - change each switch multiple times, and run any volume control back and forth many times, and see if that helps.

You could try some electronics grade contact cleaner and some lint free cloth or cotton swabs and clean the contacts on the speaker wire and receiver (Do this with the power off....)
If you have banana plugs or other connectors verify they are securely fastened to the wire.
If not, examine the wire for signs of oxidation (looks dull/corroded instead of shiny).
Take a flashlight and have a look into the top of the AVR to see if there is a lot of dust in there , that can have some odd effects occasionally.
Since the crackle follows the channel and not the speaker, that is about all you can do unless you're willing to crack open your AVR and I really don't recommend that unless you know what you're doing.

Dirty switch is also my guess. But there can be several possible culprits. So exercising anything that can produce a change in what's coming out the speakers could temporarily help. Once you find out which one it is, then a quality (like DeOxit) contact cleaner could be used judiciously.

But my experience is that once this starts happening, it will come back all too soon and you either live with the device or without it.

I work on very fussy A/D conversion units.
Cleaning often solves a lot of problems. The very fine pitch of the leads can cause a variety of problems when crud builds up. Particularly in the pre amplifier areas.
Get some canned compressed air. Good quality electronics cleaner. Blow it out first. Try and do it with the unit upside down as well. Apply the cleaner and blow it out while it is still wet. This stuff can dry real fast. Careful not to get it in your eyes or breath it. Flammable too. After a thorough clean, let it dry for a good long time. The cleaner will seep under components and sit there for quite a while. Leave overnight near a heat vent is good.
Cleaning solves a lot of our problems. But then our problems are seldom audible. Just show up as 0.0001 off specifications.

Hope you took the opportunity to pick up something to replace that 707. It's not worth spending anything on, but one thing you can try for free that might do nothing useful at all is a factory reset:
https://onkyousacustomersupportblog.wordpress.com/2011/09/12/factory-reset/

I had a 707 that died last year with the well known HDMI board capacitor failure. For whatever reason, mine wasn't covered by the recall, so I replaced it with a Denon AVR-X3200.