Suggestions re: 2ndary air inj valve
 

Hello please bare with me as I try to explain my issue.

My check engine light came on in my 07 Impala & The code showed I had an evap leak so the cheapest possible fix suggested to me was to buy/replace my gas cap which I did to no avail. I then on suggestion of a friend had the gas tank purge control valve replaced. The light went out when i drove it home but came back on when i went out the next day but with a new code (Forgive me but I dont know the specific code sorry other then began with a P) but was told it referred to the secondary air injection check valve.

The problems I've seen in my car is the very horrible gas mileage which didnt seem to get much better after replacing the purge valve & then recently 8 out of 10 starts the car sounds like a very loud vacuum comes on sometimes with a high pitched ring for maybe no longer then 10 seconds or so then it sounds like it shuts off just like shutting off a home vacuum cleaner.

So my question is should I just replace the secondary air injection check valve or should I try anything else first? The only reason for questioning replacing it is obviously the cost of the part/labor (not the cheapest of parts lol) but also I dont understand why that code seemed to occur immediately after replacing the purge control valve & if there's anything else I need to be aware of/look for prior to replacing that valve thing & if it's possible that replacing that valve wont fix the problem .... Bottom line I just wanna make sure I kinda know what to look for & be investing the money on parts in the right order so to speak.... Just some sort of guidance would be so welcomed & appreciated ... Thank You

07 Impala
mileage roughly 85,000