hello i have a 2000 blazer with 4wd it has 145,000 on it and it has developed a knock or tick but the thing about it is it comes and goes. Any ideas

 

is it knocking like this
YouTube - ‪Miata Rod Knock‬‏

or ticking like this?
YouTube - ‪Broken hydraulic lifters‬‏

if its ticking,its probably just a lifter,Its not a huge deal,but needs to be replaced or remedied. Check your oil and if its a little low pour about half a quart of transmission fluid in your oil. run it 50 miles then do a oil change,the fluid will clean up some crud in your oil. or just buy a bottle of engine flush and run it at IDLE for 5 minutes then change oil. Alot of times the lifters just get dirty and make noise.

If its a knock,could be a rod,piston wrist etc. That will mean a rebuilt is on its way. Hows your oil pressure?

 

oil pressure is between 40-45