Noise from cistern

[pooksahib]

Hi. This is my cistern:
https://www.diynot.com/diy/attachments/img_20190627_102421859-jpg.166704/
and I think the bit with the 2 red caps is this:
https://www.amazon.co.uk/Ideal-Standard-Armitage-Conceala-flush/dp/B00I461ZQ0?tag=diynotcom-21

I'm hearing a faint squelchy/whistly noise from the cistern. Oddly, a gentle tap on the wooden cover will stop it (for a short time). The noise may be coming from the red-capped flush valve - when I prodded it, the noise stopped.

There's no leakage into the pan. Can anyone suggest cause and cure for the weird noise? Many thanks if so.

 

[Madrab]

If its a slight whistling noise then chances are that it'll be the fill valve that's passing and knocking the flush valve sends vibrations through the water to the float.

 

[pooksahib]

Hi Madrab and thanks for replying. I'm afraid I don't clearly understand "...the fill valve that's passing and knocking the flush valve". Could you elaborate, please?

I should also have said that the noise happens anytime, even long after the cistern has filled up.