Recently I bought a used 2006 S-type. When I met the owner to look at the car we started it up and while we were looking at the engine it started to knock. The owner's face turned white and admitted he thought it had crankshaft bearing problems- but it didn't sound like that to me. I offered him $3000 dollars less and he took the deal. It had new tires and battery, good glass, dirty but perfect interior, no dents or nicks, perfect paint except for the front bumper. When buying a used car I like to change the hoses, filters, fluids, water pump and alternator. While removing the hoses I happened upon a 4 foot long string of a broken accessory belt that must have been flinging around causing the knock. Bumper is painted, all put back together and it runs perfectly. Car, paint and parts added up to about $2000. Fantastic car -best money I have ever spent.

 

You did a nice negotiation good job.
About the knock sound, It may be the valve bucket. I have one sometimes on cylinder 2 (middle left bank) that disapear when i hit the gas pedal.