Hello and welcome to just answer. This is a common problem on the Isuzu 3.5, and is caused by a sticking valve lash adjuster. Sometimes changing the engine oil to a synthetic type like Mobil1 and adding an oil additive like Sea Foam or Lucas oil additive will help free up the lifter(s). If this does not help, then replacement of the lash adjuster/lifter will be required. If the rest of the vehicle is in very good shape, then I would still recommend purchasing the vehicle and worse case scenario, replacing the lash adjuster(s) if needed. Hopefully the oil and additive will free up the lash adjuster and you will be good to go.
I hope this information helps, and let me know if you need further assistance with this.
It is known as a lifter, and since this is a dual overhead cam 24 valve engine, there will be 24 lifters. However you probably have one or 2 that are noisy and probably do not need to replace all 24.