I have a 98 chevy venture and it sounds like it is knocking on the drivers side under the hood. But the knock stops as soon as you hit 50. Before you hit 50 it get louder. Can you tell me what it is?

If it is a little tapping then it is a lifter that is trying to leak down overnight , as you were suspecting, the only way to fix it is to replace them but they are roller lifters and are expensive to replace, you can try some Lucas oil additive on the next oil change, put a quality 10w40 oil and 1 whole quart of Lucas engine oil additive and if it is caused by varnish build up the lucas will solve the issue, if is a worn seal it will help it, but it has collapsed internally ,nothing will help it but a new lifter.But it is worth a try sinc ethe Lucas is less than $8.00 a bottle, just remember that this stuff is very thick , so the engine needs to be warm and running when it is added to correctly blend with the oil ,and do its job correctly. do not add the Lucas without draining out a quart as it will overfill the engine oil