1999 Blazer V6 4.3L 78,126 miles

When starting the vehicle it makes a ticking/knocking noise (it has done this since September 2005). I have taken it to several mechanics thinking it was going to throw a rod but by the time I got there the noise would go away. The last mechanic suggested I change gas (I was buying Citgo) so I did but I still put the same octane rating in it (89) and that seemed to help for a few months. Now sometimes the noise doesn't go away and I've notice the oil pressure dropping. When I first bought the vehicle in 2002 the oil pressure would be around 60 while moving and drop a little below that while idling. Now the oil pressure is barely making it to the 40 while moving and drops to 20 while idling, I check the oil and have it changed regularly. I'm thinking it might need a new oil pump but I can't be sure, is there some way to test this out before spending out a lot of money for a new pump?