EDIT/SOLVED: The culprit turned out to be a faulty offbrand 5V 3A DC power adapter. I tried another adapter - incidentally, one that I use to charge my smartphone - and the hissing sound was gone. My other router - also a Raspberry Pi 4B - uses the same power adapter as the one that started malfunctioning/hissing was using, but it is quiet and operates normally. At the very least I draw the following three conclusions:
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5V 3A is enough to run a Raspberry Pi 4B as a router with constant heavy TX/RX of at least a couple of terabytes per day.
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I should be prepared for the other offbrand power adapter - namely the one that came with the OkDo bundle - to fail within a year or so.
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There are really nice people here that give quick and elaborate responses! Thank you all! ๐ฉต
The linked file is a recording of a hissing sound that my Raspberry Pi 4 started to make sometime during the last 24 hours. Another symptom is that the power on and CPU activity LEDs are out. It is not running hot. It doesn't have an internal fan. It is running OpenWRT. It has been pumping Linux ISOs in and out non stop for about four months.
I have tried cleaning it by dedusting it. I have also tried rebooting it. While rebooting, it squeals like a mouse, like it was suffering but the LEDs function properly. As soon as it loads user space though and resume routing, they turn off and the hissing sound continues.
Here are some system logs with severities warning and error . As you can see, the logs are from the 15th and I don't think any of them pertain to the hissing sound and/or the LEDs.
WARNING
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 0.196065] pci_bus 0000:01: supply vpcie3v3 not found, using dummy regulator
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 0.196201] pci_bus 0000:01: supply vpcie3v3aux not found, using dummy regulator
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 0.196257] pci_bus 0000:01: supply vpcie12v not found, using dummy regulator
[May 15, 2026, 23:29:12 GMT+2] user.warn: [ 7.225917] urandom-seed: Seeding with /etc/urandom.seed
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 8.327998] snd_bcm2835: module is from the staging directory, the quality is unknown, you have been warned.
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 8.365104] brcmutil: loading out-of-tree module taints kernel.
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 8.780919] brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43455-sdio.raspberrypi,4-model-b.bin failed with error -2
[May 15, 2026, 23:29:12 GMT+2] kern.warn: [ 8.792642] brcmfmac mmc1:0001:1: Falling back to sysfs fallback for: brcm/brcmfmac43455-sdio.raspberrypi,4-model-b.bin
[May 15, 2026, 23:29:12 GMT+2] daemon.warn: dnsmasq[1]: no servers found in /tmp/resolv.conf.d/resolv.conf.auto, will retry
[May 15, 2026, 23:29:18 GMT+2] daemon.warn: dnsmasq[1]: no servers found in /tmp/resolv.conf.d/resolv.conf.auto, will retry
[May 15, 2026, 23:29:21 GMT+2] daemon.warn: odhcpd[990]: No default route present, setting ra_lifetime to 0!
[May 15, 2026, 23:29:21 GMT+2] daemon.warn: odhcpd[990]: rfc9096: br-lan: piofile updated
[May 15, 2026, 23:29:37 GMT+2] daemon.warn: odhcpd[990]: No default route present, setting ra_lifetime to 0!
[May 15, 2026, 23:29:53 GMT+2] daemon.warn: odhcpd[990]: No default route present, setting ra_lifetime to 0!
[May 15, 2026, 23:30:18 GMT+2] daemon.warn: odhcpd[990]: No default route present, setting ra_lifetime to 0!
ERROR
[May 15, 2026, 23:29:12 GMT+2] kern.err: [ 0.307942] bcm2708_fb soc:fb: Unable to determine number of FBs. Disabling driver.
[May 15, 2026, 23:29:12 GMT+2] kern.err: [ 0.307968] bcm2708_fb soc:fb: probe with driver bcm2708_fb failed with error -2
[May 15, 2026, 23:29:12 GMT+2] user.err: [ 7.197094] insmod: module is already loaded - fat
[May 15, 2026, 23:29:12 GMT+2] user.err: [ 7.204851] insmod: module is already loaded - vfat
[May 15, 2026, 23:29:22 GMT+2] daemon.err: procd: Got unexpected signal 1
Judging by the squealing during reboot, I believe it to be either the CPU or memory, although I have never hears memory make sound, while CPUs, I have.