Ticking in washing machine when draining sink

[AFB2301]

Wondering if anyone can shed some light on what might be happening here.

Kitchen is situated in a block of flats, with a common waste stack. When draining the sink (which drains into the common stack) there had previously been an issue with a knocking noise which turned out to be PVC pipe expansion. This was resolved by installing insulation.

Now there is a new noise which is a very loud metallic sounding ticking noise which sounds like it's coming from the washing machine (empty washing machine) which happens as water is draining. As background info, it's an integrated washing machine that also drains via the common stack. Earlier on in the week there was an issue with the washing machine encountering an error due to overloading, and this ticking has been happening every since, but this may just be a coincidence. Any ideas gratefully received!

Edit to add: no water detected in washing machine or underneath - ie, it doesn't seem like a leak.

 

[Best]

Does it happen with only fairly warm water going down waste pipes? Any plastic waste pipes are better not tight against timber or plasterboard etc.
I would have the washing machine waste outlet hose disconnected first to see later if noise still there. If noise then has disappeared, it is something to do with the machine. You sometimes got to try to rule out things.
Some soil pipes have an air admittance valve and I suppose it could make a noise.