This is a common issue with the iPhone 7 known as “Loop Disease”. The reason behind this issue is that one of the audio units disconnects (in your case the battery expansion probably disconnected it, but with iPhone 7 it’s an accident waiting to happen.) and on startup the phone tries to get in contact with all the chips. (Think computers have a Power On Self Test, this is an iPhone’s version of that.) It waits and waits for the confirmation the specific audio unit works, and never gets a confirmation. At this point it will time out, reboot, and do it all over again.
You can watch this YouTube video about it by Hugh Jefferys ([https://www.youtube.com/watch?v=bguTwD6KcjI|https://www.youtube.com/watch?v=bguTwD6KcjI]) and in his case Apple replaced the phone free, you can try going into a store and see if they will do the same for you, but the actual issue is on the logic board and would be very hard to replace. If you google “Loop Disease” you will find all kinds of info and if you are experienced in that area you could attempt a repair based off YouTube or something. I would personally go to an Apple retailer / store and see if they could do anything free (first making sure the screen isn’t cracked, they would charge you to replace it even if you didn’t want them to) and if not explore other non-Apple-approved repair shops that might cost money.[br]
Good luck!