Wall Street has a few big worries about Google's results this quarter

Investors are cautious going into Q2 earnings for Google parent company Alphabet, according to SunTrust's Robert Peck.

Expectations for the company's July 28 announcement are "modest," he writes in a note to investors this week, highlighting some of the biggest questions and concerns around Alphabet's results, including weakening search-revenue trends and the effects of Brexit. (Roughly 9% of Google's revenues are from UK and more than 20% are from Europe.)

He also highlights a topic that made analysts very nervous on Alphabet's last earnings call: How Google has increasingly been spending more money to make more money.

Google's total traffic-acquisition costs - the fees it pays to partner websites that run Google ads or services - were $3.8 billion, or 21% of total advertising revenue, but the portion of its TAC that goes to distribution partners - the one-off deals that Google strikes to make sure that its search engine is the default option on things like the iPhone - popped 33% year-over-year.

"Investors remain concerned about the growth of distribution TAC which rose more than expected during 1Q at +33% vs. revenue growth of 20% (~26% ex FX)," Peck writes. "This is the 3rd straight quarter of accelerating growth in TAC."

Peck attributes the swelling distribution TAC to Google's growth in mobile and programmatic advertising, and added that a renegotiated contract with Apple could be a potential risk going forward.

Earlier this year, now sealed court documents revealed that Google paid Apple $1 billion in 2014 to use its search on iPhones.

He sees distribution TAC as "continuing a steady upward trajectory" for the future: