1998 GMC C1500 Loud rattling noise

My transmission was rebuilt in the last 10 months and has less than 1,200 miles on it. Yes thats 1,200 not thousand. Yesterday when I was headed home I started to here a very loud rattling noise, the noise is coming from inside the transmission pan. The vehicle other than the noise ran fine. Fluid is nice and red no burnt smell, shifted great. Is it possible for the filter to have fallen into the pan? A bolt? Where would the bolt come from? I called the builder he is out of town until the 17th and it's a one man shop. Thank you for any info.