You need to fix it right away, but there are probably things you could do that don't require lifter replacement. Try Amsoil engine flush. I also will take the rocker covers off and use a hollow push rod to to squirt Gumout or similar directly into the lifter. Push gently on the pushrod a few times to allow it to flow into the lifter. Let it sit awhile to do its thing. Replace the oil (I use 10-30) and see if that improves it. While you have it apart, check for worn rockers, bent pushrods, etc. Might as well do all you can while it's apart. Use an oil pressure gauge to be certain you are building pressue right away. I never shied from throwing money at something, but I always try all the other possibilities first.
Do you know why the exhaust thumps, or did that quit at the same time?
