Growling noise from steering pump

[LSUMurse]

I recently put power steering in the truck, holy crap what a difference!

However, if I start the truck when it's cold outside, the pump makes a growling/grinding noise. It stops making noise once the engine has warmed up and doesn't make any noise at all if it's warm outside when I start the truck. It's been on for about a month, so I don't think it needs any further bleeding. It's not low on fluid. It was a junkyard pump from a '76 and the gear is from an '81 so I switched the high pressure outlet to accomodate the metric thread gear line.

What do y'all think? Get a rebuilt one? Is there something I can trouble shoot?