Emblazoned

Hello All,

I've been lurking around these forums for awhile just google searching for basic answers but I can't seem to figure this one out so I figured I'd reach out and ask the experts for a little help. I've had my 03 mustang for about 5 years now. Love the girl to death. I recently had her tuned up really well due to an exhaust issue and my clutch started to give out. For the past few days, I've been hearing a strange ticking noise. It sounds like rocks are hitting on the exhaust pipes on the bottom of the car. I read online that this issue could be bad spark plugs which my car is definitely due for some new ones. I'm replacing them myself but could only get one done tonight because it got too dark out. I was wondering a couple things. 1) How can you tell if the spark plugs were installed correctly(I'm a noob at spark plugs sadly and I'm afraid, don't want to ruin my car) 2) If it's not the spark plugs then what do you guys think it is? An exhaust leak maybe? The car sounds fine when idle and doesn't seem to have lost any power at all. I just drove it today and its running fine and smooth. I'm just afraid because I don't want it to develop into a much worse and expensive issue down the road. Please let me know what you guys think. Thanks