AppleTV 4K audio distortion

My kids gave me an AppleTV 4K for Christmas. After setting it up and watching it for awhile we noticed the tv speakers distorting. It sounds like a rattle or vibration in the speaker itself. I installed a new 4K HDMI cable and have tried the different audio settings. This has not resolved the issue. The distortion is not audible at first but arises about an hour or so into viewing. The TV itself is not 4K but 1080p. I've checked connections and restarted devices but the condition persists. I also have a Roku 4K unit connected to the TV which I've used for several years with no issues. I see others have written here in the forum about audio issues which may be similar to mine but it's hard to tell from the descriptions. Advice?