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:

1. 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.

2. 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.

3. There are really nice people here that give quick and elaborate responses! Thank you all! 🩵

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.

Have you visually inspected the board for damage?

I have not. I'll do it first thing in the morning. Thanks! 💯

sounds like coil whine to me. if it started out of nowhere, it could be because of metal fatigue. if the leds are out, could be a cracked solder joint.

i'm seeing warnings about the pcie regulator, how much are you running off the usb ports? also what is your power supply rated for?

EDIT: Pasted wrong link to power supply. Corrected!

Never thought I'd hear "coil whine" in any other context than GPUs. 😅 Also, "metal fatigue" was a new concept to me, which anthropomorphizes the Pi to such an extent that I feel bad for it. But I'm hypersensitive.

I am running one USB-A to Ethernet dongle off of one of the USB 3 ports, nothing else. I just checked the LEDs on the dongle, the Pi's Ethernet port and on my switch that ties a lot of other stuff together before it all goes to my ISP's fiber box (forgot the official name), and they are flashing with that rhythm that usually indicate Gbps speeds.

The power supply is a 5V one: https://us.rs-online.com/product/okdo/2012167/72127392/

Tomorrow, I'll actually open up the chassi to check the board for defects.

Thank you very much for your reply! 🙂

Sounds line the pixies in the wires are getting upset and preparing to let out the magic smoke

Coil Whine, or your heatsink wasn't seated properly and you have air gaps.

I concur with your diagnosis. I can't seem to recall to correct incantation that soothes the pixies' pain, though. You don't happen to have the book Raspberries - a Pixie's Perspective at hand?

I'll check it out as soon as I wake up! Thanks! 😊

hm, 3A should be enough for the Pi4... barely. it's theoretical maximum draw is 15.3W, so if you're maxing it out a lot you could be seeing brownouts. doesn't sound like you are though.

also, if you think metal fatigue is wild, wait till you hear about tin whiskers.

Hissing vs Whine. Hissing sounds like a high voltage thing. This can even happen in low voltage systems. You should be able to smell that, smells like a very dry carpet. Whining is just the coils getting old. HV can cause sparking and fires, so please check that.