when starting my car it sounds like a hard loud knocking - she never did this before - it has 106,000 miles on it and had the head gasket replaced in 2014. Consist with oil changes every 4-5 months since I dont drive 3,000 miles every 3 months - could you estimate how much it would cost to repair since it is a 1992 Plymouth Sundance?
Until the source/cause of the knocking is diagnosed, no one can make a guess about repair cost.
Find an independent shop to diagnose your problem and they will provide an estimate. You may have to pay for the diagnosis, but they may waive the fee if repaired by them.
If you trust the shop that did the head gasket, ask them.