Exhaust pinging

Hi everyone- one more new guy with van problems. I just bought my first Ford van, a 98 E250 with 132000 miles and a rebuilt engine. After I run it for a while and shut off the engine, for about 3 minutes the exhaust makes a pinging or tapping noise- much like the old style home radiators do when they are cooling down. Is this normal? A friend of mine (who also owns an e-series van) told me not to worry but I am not so sure. Does this happen on anyone elses van? What is this from?
Please help if you can. I am new to this!
Thanks

It should not be a concern, but you may want to take a look under the van and make sure that all the exhaust hangers are still there and properly attached, especially the one right behind the Y pipe . For some reason that one is prone to breaking off which puts a strain on the connection at the Y pipe. That pipe is EXPENSIVE as it has two oxygen sensors and a catalytic converter built into it so you don't want to mess it up. Mine eventually cracked there and would not stay welded until the hanger was replaced. Most of my under the van noise was related to vibration due to the broken hanger and the cooling down of the converter (which is probably what you are hearing).