Name that noise! Seriously, fans are being noisy and “tapping” them gets them to be quite. What is that noise from? Is the fan going out or is there an easy fix (other than tapping on them every other minute)?
Probably misaligment and cheap or no bearings
How old are they? Fans on moving heads tend to wear faster due to start-stop motion. Note, fan itsefe doesent wear, the bearings are the weak link in this kind of applications. When finger tap wont help anymore replace the fans. You might want to consider maglev fans then.
If not worn bearings as stated above :…
pull off fans run them. is the blade tweeking the heat sink?.. looking at the armature as you hit it. maybe its bringing it back on its magnetic seat its floating on… adjust mount or blade and power up so as it applies vacuum to its plenum it will not jump its seat and let itself hit the heat sink… I love diagnosing things just by sound…HA HA to sound boarding power…
paint them and see where wearing off if that will not apply…
+almost all of the commenters: yeah bearings were my first thought, I’ve had fans go out on computers that sounded similar right before going out (but not not quite the same noise). These are semi-new, maybe 6 month old printer, not used for periods at a time (due to lack of disposable income to spend on filament lol). Just hoping they weren’t going out this time; thanks for the ideas on what else it “could be” to give me hope I don’t have to go buy something and wait around for it to get here 
@Caleb_ABUBANDBEYOND the blades aren’t close enough to heatsink… Although, that does give me an idea, I wonder if one of the wires is rubbing…
I just noticed, the bottom of the fans look kinda yellow, wonder if they’ve always been that way or if it is just lighting or the heat is doing something… I’ll have to look at earlier pics to see! (sorry just talking out loud hah)
I get the exact same thing happening with the Mendel… thought it was just me
Definitely bearing… this fan has no ball bearings, it has bushings.
And when they become worn, the fan axle first sometimes then more often, doesn’t turn in this bushing it is rolling against the bushing wall which is some wider than it should when it’s become worn. So it is rolling instead of gliding. And this is making the sound.
I hope this is understandable.
resonance
First things first: Since you’ve only secured one side of the fan (the bottom 2 screw holes on each fan), the other side is free to move. This means that 1. Heat can bend the plastic frame ever so slightly (which could lead to it being unbalanced), 2. The fan shroud can act like a sounding board, making any vibrations audible. If you can put screws in all corners of the fan, you might reduce the noise being so loud.
If you have some sewing machine oil, and you suspect the bearings are noisy, you can “usually” lubricate the bearings. There is usually a sticker on the rear of these fans, and under that is the bearing. Sometimes it’s under a little rubber cap that you can remove then replace. Once it’s exposed, you can put a tiny bit of oil in there. If not (if it’s sealed - some cheaper fans are) then your only option is to replace it.
I tend to use a needle eye to pick up a tiny bit of oil (the eye holds more than a drop on the point) and drip oil on the rear of the bearing shaft a few times (depends on the needle, size of the motor), then put the sticker back in place. If you get oil over the back of the fan casing the sticker will not adhere, so if you have a spill on the fan body where the sticker goes, clean it with isopropyl before putting the sticker back in place. You want the sticker in place as it stops the oil leaking out and/or slowly evaporating.
The oil may take a little while to run in, so don’t expect it to be immediately silent.
PS: DO NOT use RP70/CRC/WD40. Same goes for Silicon-based lubricants. They will work very briefly then completely evaporate, leaving absolutely NO lubricant in the bearings. If you haven’t got sewing machine oil, a tiny amount of car engine oil is also an option.