12v pinging noise

I have an 91 with 202xxx on it. ever since i bought it it has had a pinging noise my other 12v does not have it at all and it has 243xxx on it. i thought it was something with the ac compressor, but i took the belt off and ran it and it still does it. it seems to be coming from where the oil fill is. Its weird because it doesn't start right away when u start the truck there is a 8 to 10 second delay. does anyone know what it is??


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