Another, maybe bigger, problem is probe requests. A client device, e.g. smartphone, uses "active scanning" where it sends probe packets and asks "is there any A

Another, maybe bigger, problem is probe requests. A client device, e.g. smartphone, uses "active scanning" where it sends probe packets and asks "is there any AP in this channel?" instead of "passive scanning" where it would wait for beacons. Active scanning has the advantege of being much faster. In passive scanning, client needs to stay 200-300 ms in each channel and that ads up quickly when you consider 2.4 GHz and 5 GHz channels. So clients prefer to use active scanning to quickly discover APs. Some clients send periodic probe requests even when they are connected to an AP in case they need to switch to another AP. All these probe requests together with beacons pollute 2.4 GHz.