Beating the phone against your hand is only a temporary fix. The issue is with a part that holds the Taptic Engine in place. When the part gets loose/dislocated, the rattling sound occurs. I think it’s best to just take it to an Apple Store and have them fix it. I took mine to a third party store (because I don’t have Apple stores in my country, and the authorized service centers don’t fix these issues), but my phone’s fine now. It was a really simple fix.