What chemicals are in tap water that gives it a slightly basic pH level
The tap water is likely "hard," i.e., contains some dissolved mineral salts, most likely (primarily) calcium and magnesium carbonates and bicarbonates. The anions of these salts are slightly basic, consuming $\ce{H^+}$ ions and thereby raising the pH. Carbonates (with the obvious exception of alkali metal salts) are only sparingly soluble in water, while bicarbonates are typically very soluble. A couple of other subtle factors may affect the pH:


Lower atmospheric pressure encourages the evaporation of dissolved carbon dioxide from water. Since dissolved carbon dioxide exists in equilibrium with carbonic acid, it would tend to lower water pH. If you live in an area with comparatively low atmospheric pressure, your tap water might have a lower concentration of $\ce{CO_2}$, which would favor higher pH.
At higher temperatures, bicarbonate decomposes to water, carbonate, and carbon dioxide. Carbonates are relatively insoluble, and the consumption of bicarbonate should drive forward the dissociation of any dissolved carbonic acid, so a higher temperature should have the effect of lowering pH.