Hi all hope all is well.
I have just noticed that there is an intermittent signal in all my amps Naim and mission. It does not appear in the speakers when the system s are on its only present in the amplifiers and It only happens when the washing machine is in cycle short bursts when the drum is turning any ideas .
Regards
Anthony
Is it still present if you some cymbals in with the drum? 
Ha it a humming noise from all amps and only happens when the washing machine is on. I was wandering if the mains filter in the washing machine was on its way out.
I’ve been experienced the exact same issue here in the states. I would advise to try a Tripplite IsoBlok Ultra. That’s what fixed it for me in a rental.
Depending on the issue, it can be the motor leaking back onto the mains or static charge back through earth. Both are very common.
If the hifi and washing machine are on different circuits, I’d suspect the static over earth as safety earth is permitted to (and often does) cross circuit boundaries before going back to the ground busbar.
Any hifi that links signal ground to mains ground is liable to have issues if the mains earth is polluted. Condominiums are the worst for this.
[addendum] rereading your post, it sounda like you say the amps mechanically hum but only when the washing machine is on. In which case, the motor or heating element would be distorting sine wave and giving you some DC offset.
Could it be DC on your mains line? Funnily enough John Darko discusses this in one of the latest episodes of his podcast.
There’s no such thing. A DC offset is an asymmetric AC sin wave. Like +120v over -110v.
But it’s usually transient and associated with something that malforms the wave like a motor.
Mains filter in a washing machine… HAHAHA
The mains filter on a washing machine helps prevent the electrical interference, that is generated by the motors, from passing through the cables and disturbing other devices
Not exactly the same but we had interference from our Miele WM, turned out new brushes on the motor fixed it, our latest one is brushless so shouldn’t happen again, good luck.
Martin
Fixing the problem rather than adding a sticking plaster fix seems far more sensible to me. I would just get the washing machine fixed.
It’s a motor with variable speed. They will all in some way put dirt on the grid.
You could try upgrading the motor power supply. Or replace the captive power connection cable for OFC silver plated.
Washing machines often have, as you say, a built in mains filter. It’s possible that this, rather than the motor, is defective. It’s probably easy to replace, and maybe free if your machine is under warranty, if not it’s usually a simple DIY job.
Switch to using a laundromat 
Couple of years back our gas boiler broke down I used some electric oil filled radiators in my daughter’s bedrooms, when they tripped in and out on their thermostats it could be heard through my system.
Due to the intermittent nature of the noise it proved pretty frustrating to figure out what was going on.
Fortunately the boiler repair was inexpensive and back up and running later in the week
So I had some weird transformer humming noises recently, thought it might be related to interference from the household appliances. I tried a DC blocker on the mains lead into the power amp (iFi DC Blocker+) and that solved it. If you have identified the source as the washing machine then if there is some way of addressing that, prevention is better than cure!
DC blockers do work well. Use two iFI DC Blockers on my NAP 250 that used to hum horribly so.
Virtually stored all the hum with no adverse effect on SQ.
DG…
???
Martin
Or indeed, stopped.
DG…