Bob Sanders,
You can find the relevant rule in Article 12 (Amateurism) of the D1 manual.
12.2.1.2 Tryout After Enrollment. After initial full-time collegiate enrollment, an individual who has eligibility remaining may try out with a professional athletics team (or participate in a combine including that team) at any time, provided the individual does not miss class. The individual may receive actual and necessary expenses in conjunction with one 48-hour tryout per professional team (or a combine including that team). The 48-hour tryout period shall begin at the time the individual arrives at the tryout location. At the completion of the 48-hour period, the individual must depart the location of the tryout immediately in order to receive return transportation expenses. A tryout may extend beyond 48 hours if the individual self-finances additional expenses, including return transportation. A self-financed tryout may be for any length of time, provided the individual does not miss class. (Revised: 1/10/92, 4/24/03, 5/26/06, 4/26/07 effective 8/1/07)
D1 manual