if it has never been overheated or run low on oil (or really long intervals) it shouldn't be rod knock.
check the timing belt tensioner - they fail all the time and they will knock. i wouldn't drive it much if the tensioner fails this is an interference engine and you'll have bent valves (more money than a headgasket repair).
i replace those tensioners as a rule, they are failure prone anyway. they will slap hard and sound like rod knock as the metal on the tensioners slams up against the metal of the engine. you can check by listening closely with a stethoscope or possily a rod/screwdriver on the timing cover while runnning. if it's louder on the drivers side it's the tensioner. you can also pull the timing cover (possibly just the drivers side) and see it slapping up and down.
if you go in there again do it right and get a complete timing belt kit if it is the tensioner failing. $160 on ebay for all new pulleys and tensioner. a new belt alone is silly with old pulleys and tensioners that have a ton of miles, years, and are devoid of grease in the bearings. maybe skip those on an old rusty beater - but not one you just dumped a buttload of money into for a headgasket repair and expect another 100,000 miles from.
often happens after a timing belt job too, old part that's prone to failure anyway, it doesn't like being fully extended and recompressed, extended again.
again - replace it immediately and don't drive it - if it slips time you'll have bent valves (unless you're really lucky which you aren't feeling right now).
piston slap is obviously - loudest at start up and gets better as it warms up usually.
Edited by grossgary, 19 July 2013 - 05:04 PM.