Hmm, the best way to check if it's a bomb is to look under the bonnet. You're looking for any sort of clock (analog or digital, although digital is favourite now) that will be counting down to 0. This is when the bomb explodes. It will also have about 3 wires coming off, usually red, black and one other, maybe green. You must decide which one to cut to disarm it. It's customary to to sweat a lot while doing this and tell your wife you love her. When you've made your mind up, go to cut it, then suddenly change your mind and cut the one next to it. This will ALWAYS be the right one. Don't ask me why, it just is.
Or it could just be your tappets, does it get any better when it's started to warm up?