I work on my own bikes most likely on this new of a bike its probably something simple as in a bolt not torqued down or something else that has worked itself loose. the cost of the bike should not have come up at all. I have a fuji nevada its about the same bike as the marlin 5. whats at issue is a possible safety issue if its a loose bolt or spoke not in tension etc. the mechanic should have went through the bike checked everything enough to at least know where the squeak is at. if its that the coil fork has a squeak might not can do much about that. Friend of mine has a marlin 5 his spokes started loosening up after about a year tightened them and his squeak went away. @OP also might be helpful to tell them under what circumstances it squeaks. when you brake when its rolling or when you are turning. all of which would be helpful to limit to where the problem is at.