97 Boxster "Thumping"

[KConn23]

Hey guys, I have a 97 Boxster I'm getting ready to sell after about 6 years of ownership. For a while, I wasn't driving it on a regular basis, so I was going out and starting it weekly to keep it in running condition. I noticed that the battery was draining on a regular basis, often after only a day or two.

I put in a new battery and as soon as I touched the positive lead to the batter I got a loud "knocking" sound from the back end of the car. I went ahead and attached the lead and went back to determine where the noise was coming from. It's under the trunk section and it sounds like a jackhammer it's so loud. This is without the car even running.

Any ideas what this might be? Car fires right up and runs with no problem, but the knocking continues and does not change.

thanks,

Ken