Your noise is not due to the guides unless the valves are bouncing off the pistons as a result. I've seen a few so loose that this happens. It won't go away (and won't run afterward).
Your noise is probably a worn rocker arm or cam lobe since that motor doesn't have hydraulic lifters. My original advise still holds. There is no reason to remove the heads untill there is a verifiable reason. Remove all the rocker arms - find the bad ones - replace them all (when putting back together), pull the plugs - find the worse carboned - add air and remove the valve springs - evaluate the guide - does it have a good groove at the top outside (seal uses the groove to retain grip) - is it loose in the head - does the valve have inordinate play (this is not critical as the seal will adapt for a deminishing time depending on play) - replace the seals - do the rest of the valve seals - if you find a loose guide or one with the groove worn off (the early motors has hard teflon seals that could wear out the brass guide if they rocked around too long) then the heads must come off.
This process is relatively painless and will almost always last for years. If you have to replace a guide, then of course all the guides should all be replaced. But, have NO concern about the bottom end. These motors never use oil through the rings. (and I'm a person who never says never).
Bosch Master, ASE Master, L1
33 years MB technician