I had a similar problem particularly on starting with the knocking and vibration. My mechanic said it was the supercharger. This became progessively worse over time. I finally ordered a rebuilt supercharger and my mechanic installed it. Problem solved. Check with your mechanic. If he says it is supercharger email me at swcover@msn. Com and I'll give you the name of the company I bought mine from. It ran $375 and included return shipping of the old supercharger. I replaced my supercharger at 370K, now at 387K.