Blazer knocking
 

So i got this 2004 chevy blazer from my fathers girlfriend it had been sitting for 3 years and found out it had a knock we investigated and found there was a pushrod just gone for some reason so we put one in and let it sit for the winter a few weeks ago i start it and move it and let it run and it runs fine the next day same thing start it and warm it up like 5 min in it starts silently knocking then it gets louder have no idea why it would do this any tips to get me started in the right direction would help