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These are the questions that are very difficult to answer. It's the old, "wish I could be there" thing. All I can recommend is that you take it to your regular shop and let them listen to it.Any dealership or private owner will not balk at that, and if they do move on.
Most shops have a comprehensive used car inspection program that will go over the car with a non biased opinion, and cost under $100.00
If you really like this car, and you have verified that the drive train warranty is transferable, than I would get the car looked at.
Now, what could it be? Rapid tick/tick... Upper engine issue, hydraulic lash adjusters, dirty oil causing HLA's to clog. Slower knock/knock, lower engine...
Get it looked at before you buy.
Hope I helped.