pop once in a while from underneath 420sel

[SCOTTFISH]

If I have posted this again, forgive me. I could not find any information on the subject from before. I have 90 420sel (w126) that at 110k had a timing chain service done. I now have 116k on it and all of a sudden have developed a strange noise or pop. After driving, I pull into a parking place and put into park, but leave the car running. With the window down, once about every 1-2 minutes you can hear a pop-like noise that seems to come from underneath the car. It is fairly loud and is only a single pop. I think I emailed someone on this once and got some information, but can't remember or find the response. The pop sounds almost like you have taken a hammer and hit a piece of thick metal. It does not echo like hitting metal does and it is a very short destinct pop. I have even noticed it happening once I have turned the car off and have gotten out and shut the door as I am walking away from the car. You can sometimes hear it then until the car has cooled off for a few minutes. Could this be something to do with a heat shield or something that heat effects? If it only did it when the car was running, I would think it was something in the motor, but it has done it a few times after the motor has been turned off. Any help you can give me would greatly be appreciated.