If it seems loose that's a good indication of an issue. It's easy to hide stuff with siding - aluminum or vinyl - but there is no way to tell for sure without removing it. I have an old house as a rental property built in 1880. I had to drill through for a dryer vent. The exterior is aluminum siding and never thought I would find what I did under it. From the outside in:
- Aluminum siding
- 1x2" shims
- Tar paper
- Stucco
- Lath
- Tar paper
- Wood Siding
- Insulation
In general as long as the siding seems tight to the house and seams I wouldn't worry too much. Sure there can always be something covered up but I haven't really run into many issues with aluminum siding or siding in general. I would be more concerned with the furnace, A/C, plumbing, electric, foundation, etc.