I bought my truck from my dad with 56,000 miles on it. I was recently talking to him about the last service and he asked me if I ever heard something that sounds like a smack under the hood after it starts? I hadn't and didn't think anything of it. Two weeks ago the family jumped in the truck to come home and I fired it up. BAM! Sounded like someone punched the firewall! The wife even said "what the h@#* was that?" I jumped out and looked under the hood and saw nothing. Truck ran fine. Well today at lunch it did it again but this time I could feel it in the floor. Any ideas? Still runs fine and can't find anything out of the norm.