Bubble formation when aluminium reacts with copper(II) sulfate solution
As Ivan Neretin noted in a comment, your supposition is absolutely correct.  The aluminium metal is reactive enough with water that it forms hydrogen gas and aluminum hydroxide (or other aluminum oxyhydroxide species) where it's exposed to the solution:

$$
\begin{align}
\ce{6H2O + 6e-} &amp; \ce{-&gt;~ 6OH- + 3H2\uparrow} \\
\ce{2Al(s) + 6H2O } &amp; \ce{-&gt;~2 Al(OH)3 + 6H+ + 6e-}
\end{align}
$$

Overall reaction:

$$
\ce{2Al(s) + 6H2O -&gt; 2Al(OH)3 + 3H2\uparrow}
$$

If the $\ce{CuSO4}$ solution is acidic enough, the $\ce{Al(OH)3}$ rapidly dissolves, exposing a fresh aluminium surface, ready for further reaction.



On the other hand, the redox reaction of $\ce{Fe(s)}$ and water is not favorable enough for hydrogen evolution to occur, which explains the lack of bubbles you observed.  The single-displacement reaction between $\ce{Cu^2+}$ and $\ce{Fe(s)}$ is quite favorable, however, leading to paired reactions like the following:

$$
\begin{align}
\ce{Fe(s)} &amp; \ce{-&gt;~ Fe^2+ + 2e-} \\
\ce{Cu^2+ + 2e-} &amp; \ce{-&gt;~ Cu(s)}
\end{align}
$$

Overall reaction:

$$
\ce{Fe(s) + Cu^2+ -&gt; Cu(s) + Fe^2+}
$$

This results in the rapid exchange of metallic Cu for metallic Fe on the iron surface, leading to a film of plated copper on the surface.