327 Chevy v-8 Knock/tick noise

Over the past 3-4 months my 327 v8 in my 64' belair has developed a knocking/tapping noise, about a week ago we took the car over to a friends/mechanics garage and took the valve covers off and none of the lifters were broken so we adjusted all the lifters, added a little bit of oil was a little low, but it still has the knock

It will mostly do it at lower rpm's because I haven't hear it at higher rpm's and it will mostly never do it at idle, idk if it may be not heavy enough oil or something else and the motor has 350 heads and a mild cam if that helps...and I still drive this car every day back an forth to work

Would appreciate a quick response because I have a guy who is wanting to trade for my car in the next couple days

If this description isn't clear enough and you have more questions just let me know....

Thanks Alot Guys,
Robert

Ticking is different from knocking.. You could have a loose flex plate. Knocking usualy comes from the pan area and is a stronger noise than a tick.. Good luck in finding it.