Dylan,

The information you provide leads me to believe that it is important to find the source of this sound immediately. Any tapping or knocking noise that increases in intensity with load (and the repetition rate follows RPM) is indicative of a rotating assembly problem. That is to say, cam or crankshaft bearing related. It may not be as drastic as that; but given the mileage and description you provide, I think it may be.

Try to localize the sound. If the sound is coming from the lower end, it is most likely a big end bearing. If the sound originates from the valve cover, a cam bearing. If the sound seems to be coming from the timing belt cover, it might be the water pump, a loose pulley, or a bad tensioner.

The jackshaft bearings might be making the sound. I don't know how to ID that sound, never having heard it myself. If it is the jackshaft, I would assume that the sound would be coming from the mid-block area and probably more intense on the left side than on the right side. I don't see how the jackshaft bearings would be effected by load though. Maybe someone who has actually heard a jackshaft bearing can help here.

One other possibility is a scuffed piston skirt. That seems unlikely; a piston slap will get more intense on overrun (when coasting) rather than when loaded.

Once you know where the sound is coming from, the next step is up to you. I don't know how much experience you have with engine R&R but my guess is that you will soon be faced with that job.
--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL