Hi guys I have a 99 accord ex 2.3L .

Mechanic replaced head gasket. Now I have a check engine light and there's a frequent knocking sound- you hear a distinct "plop" every few seconds. The sound is accompanied by jolting feeling (when you're sitting in the driver's seat) and you can see the actual engine jerking. This happens while it's on park and especially when you hit the gas pedal lightly. I don't know if it happens on Drive, since I haven't driven it around since the car started acting funny.
Let me also mention that while it's on Park, the engine sometimes revvs up and down, bouncing between 1k and 2k RPM's endlessly. Mechanic says it's due to bad battery, and I agree it is bad. Also, the flex pipe is bad- it has a hole and it has a leak. But that might be beside the point.

Mechanic says the check engine light is natural for any car which had engine parts changed (I find this hard to believe). Last week, I took the car to AutoZone and did a free diag. I got 4 codes, all engine related, and any one of them can be causing the code.
He says the knocking/jolting is coming from old spark plugs. I have no doubt that the spark plugs are old, but what concerns me is that the knocking only started coming after he replaced the head gasket. (Again I find what he says hard to believe)
He is charging me $95 (parts+labor) to change the spark plugs.
Also I can't do any DIY that you guys recommend on the car right now, because he has my car right now since he is gonna replace the spark plugs tomorrow.

I'm getting second thoughts right now about the cause of the knocking, and I would hate to spend so much money on something which won't fix the problem. Again, I will repeat that the knocking came only after he changed the head gasket.

Guys what are my options? Please help!

 

I would find a new mechanic.

Post the codes you found on here.

If the engine was reassembled properly, then you should not get any codes.

Your problem could be the spark plugs. How many miles are on them?