Chromecast firmware 1.36.140335 , 3rd gen. Always happens during adverts, never happens during games.
Thx for this info folks - we're checking this out and will post back when we get to the bottom of what's going on
1.36.140318 firmware version here
Only heard them during the football. Last nights FA cup game (Tranmere v Spurs) was especially bad. Again only during the advertising.
Watched some of the live Australia cricket about 5:30 and don’t remember any noise but then again the adverts are brief and only between overs
It doesn't affect any live part of the stream or the BT Sport intro and outtro videos. It happens during the advert breaks whilst the ads play. It occurs in the middle of ads not necessarily as they change. It happens sporadically but consistently during every advert break. There is no visible effect on the screen.
My experience might help with @JamesD_Sport's investigation because I use two TVs with different Chromecasts. The 1st gen (like a memory stick) with firmware version 1.32.141268 is fine while the 2nd gen (a disc and strap) with v1.36.140318 behaves as described above. None of the other contributors have specified an issue with a 1st gen model, so it looks likely that the problem lies with the later one.
(For what it's worth, I have another issue with the 2nd gen model in that it suddenly refused to work on one of the TVs; the older one is happy with either.)