You're very lucky to live in a relatively noise-free area then!
I think you've misunderstood. I was mainly referring to my VHF/UHF scanner, which seems to have its own onboard noise source. An HF receiver behaves quite differently. If I disconnect the antenna from my R75, for instance, the bands are dead quiet. Reconnect it and up comes the noise. It's the same with my Sangean portable if used with an external antenna. If I disconnect that, the bands are only dead quiet when the whip is not extended. Extending the whip increases the noise, but not as much as an external antenna does.
The scanner, however, has very high background noise at all frequencies, with or without the antenna, unless a strong signal overrides the noise. That's obviously the reason for the squelch, but what I don't understand is why each of the four VHF/UHF scanners I've owned behaved the same way, even the Radio Shack Pro 2001 which I had back in the 80s. That was the one with 16 programmable channels, each with its own pushbutton to toggle it in and out of the scan cycle. The buttons were in a horizonal row with an LED above each button to show when the scanning passes that channel. This, of course, was a desktop scanner, because there had to be room for the keypad and the frequency display, as well.
Unfortunately I am plagued with a wideband noise from about 8.2 to 11.8MHz which raises the level by about 12dB and it's there for 24/7. It's not coming from my house as it's still there in a power cut - I suspect it's from a telephone sub-station about 50 yards away - which also radiates a large signal on 13.560MHz which is an ISM frequency around here so I can't do much about that.
We're more or less in the same boat, although my noise sometime covers up the 40m ham band. From roughly 11 to 15 mHz there's a new noise source, which I can't identify. It's a rushing sound that seems to switch on and off at 1-second intervals. It's very strong, so probably in my vicinity. That ISM frequency is sometimes active here too, and in my case it may be from the shopping centre across the street, possibly from the point-of-sale terminals.
But to return to the topic, I was originally referring to VHF/UHF scanners rather than full-fledged communication receivers.