Noise coming from subwoofers and centre channel

It is risky to use a cheater plug.  The correct answer here is you want an HDMI isolator, and one way to achieve this isolation is to use HDMI to Ethernet converters...

https://amzn.to/3OyueZU

PS - I did find direct HDMI to optical isolators at around $750 a pair, so the options above are a lot cheaper.

Also, double check you don't also have a coaxial cable coming in somewhere, those are often hum sources, and relatively easy to fix.

yes, in fact, there is an unconnected coaxial cable, which was used by a former TV provider several years ago. Do you think there may still be signal even though I haven't been with this provider for years and have never had a ground problem in 10 years?

OP: The coax only matters if it’s plugged into a device in your rack and can create a new ground path. Coaxial ground loop eliminators are cheap and plentiful. Honestly everyone should use them just in case.

Having an antenna or cable TV cable grounded outside your home by a grounding block is code,and not by itself a problem. The problem is when that cable is connected inside to a set top box and that set top box is connected to your stereo. The outer shield in the coax now creates a very long and separate ground path.

By the way, we don’t have the same issues with Ethernet because Ethernet is inherently isolated. Mostly. For this reason we don’t have ground loops when your cable Internet is connected to devices inside via Ethernet.

PS - If the coax isn't used, make sure it's disconnected outside.  Even though there may be no equipment at the provider side that long coax can function as an antenna and bring in lightning surges into the home.  Leave the grounding block in place.

One thought — you didn’t mention if you’re using a power conditioner/surge protector, which I would hope you are given the cost of your system.  If so obviously the conditioner would be plugged into the wall with a 3-prong plug so wondering if then using a cheater plug on the projector is a non issue?  If you’re not using a power conditioner that’s definitely something to address, and depending on how its designed may also help alleviate your noise issue without a cheater plug.  Just some further thoughts FWIW and glad you’ve at least identified the culprit and gotten rid of the noise — that’s gotta be a huge relief!