1999 Oldsmobile Intrigue Lifters

I have a 1999 Olds Intrigue that has 152,000 (approx) miles on it and it suddenly has a lifter noise. I changed the oil in it, (I use a high mileage synthetic due to driving conditions) but the noise is still there and I'm concerned that the car is about to go. I've talked to a mechanic and he thinks I may need to replace the lifters, but cost-wise, I might as well go ahead and just install a new motor.
Also, could this be an issue of engine sludge??