I would look for something leaking onto the exhaust first. The "every 5 seconds" comment leads me to think that perhaps there is a coolant, or oil leak that is dripping onto the exhaust, and creating the puff. Of course, it could be something more serious, but I would definitely start with the simple stuff.
If you can't seem to find a small leak, or evidence of one, then you'll need to dig deeper to see if it might indeed be a head gasket or something like that. Good luck!
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Joel Adams
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