I will comment going by the additional info you just gave.
iPads & iPhones will periodically get hot when performing certain tasks. Even though you are "just browsing" on the iPad opposed to "editing 4k video", It's been known that certain websites can tax any device (PCs included) even more than editing video. The website may be incorrectly built or have some sort of background process running.
Try to pinpoint if it's a particular website that causes this. If one website causes the issue, then something is up with that website.
Also know that iPads and iPhones will frequently run background tasks on their own like housekeeping, file management, etc. These background processes combined with other processes going on can cause the heat you describe. As long as the iPad is not too hot to hold comfortably, it should be fine. As you've described yourself, it's happening about every 2-3 days and only for 30 seconds. I personally don't see an issue with that.
As far as the sound you've described, since we have no ability to hear this sound or pinpoint where exactly it's coming from, I can't comment on it.
See the following Apple Help Topic for more info:
If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support