That sounds a bit "tappy" to me to be a big end, more like a broken piston ring or a little end (gudgeon pin end). A big end is more of a knock.
I'd do a compression check to see if a piston is reading low (broken ring) or at least pull the plugs to see if one is oily. And if you can, get under the bus and with a screwdriver pressed to your ear (like 2 of us mentioned previously) touch it onto the body of the head, maybe near the exhaust pipe, and see if the tap is louder for one piston over the others. A top end refresh is so much easier to address than a full stripdown as piston and barrel sets come in at very affordable prices compared to a recon lump and you might just get away with a set of rings. Do some checks and let us know what you find...