Fellas, putting all the hilarity and OT aside, being the wet blanket, this would be AHB's Kits & Extract board, so presume that the OP is not mashing with his rainwater but most likely diluting tins of goop with it.
I would urge caution using rainwater in a kit scenario as the microbiology may be unkind. IIRC its been done to death here before, but the take home message is that some treatment (eg. boiling it, Campden tabs etc.) may be worthwhile but many brewers use it straight from the tap without any problems.
A simple experimental batch without any rainwater treatment would be a pragmatic way to find out- if it's satisfactory and not infected then there's the answer, if not then it is time to work out if the infection is from the rainwater or something else perhaps (ie. a process of elimination).