The are some V6's that a headbolt will back out & cause the symptoms of a head gasket issue, not noise. It's been the vans & trucks though.
An after timing belt growl/clatter is usually simply that they either over tightened the tensioner or neglected a bolt all together.
I'd say IF it has a broken head bolt that it has nothing to do with the service, just bad luck. None of the head bolts are messed with & you NEVER torque or check the torque of a head after it's been installed. It will mess up the gasket!!
At my prince's dealer you would be charged to find the problem, if it were a tensioner/tech screw up, then no charge for repair & refund the diagnostic charge....technically never actually charged. If it were a broken head bolt or whatever else found, then it would be customer pay.
If the "knock" started a few days later, then why would they not "let you drive the car off the lot in THAT condition".... Until the noise, no one would've known of a condition...... I'm confused.
If it goes to investgation of a serious nature, there are "forensic" techs. They have been used for settling arguements...but I'm pretty sure it's customer pay & I have no clue how much or how they're contacted. I've heard of them, but no experienced them. You may have to send it to SoCal or something...... This doesn't sound like conditions that would merit it. I think it's used primarily for accidents.
I will say that in 25+ years my prince said he's only seen "broken" head bolts when brittle from getting over heated & is usally in for getting the head replaced or redone in some way. The alum. would give before the bolt in nearly any circumstance by pressure.
Over tightening the tensioner wouldn't hurt the head. It would be noisey! Removing the valve cover isn't the head bolts.....
The rate of dishonest/undertrained techs at a non dealer shop is actually higher accoording to a report on dateline & another on 20/20 a few years back. A non dealer you only have a 25% chance of proper repairs the first time. At a dealer it was 65%, I think..... Your best bet is to create a relationship with your tech no matter where he works. They're usually more honest..... maybe not more trained.....but anything helps!!
I'm confused.