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2008 Chevrolet Impala - Starting issue

Ray…love your column…I have an engine problem,when I start
my car, when it has been sitting for more than 6 hours…
mostly in the morning at first start & when I go home from work after 8 hours. There is a noise…sounds like a siren…really…the engine starts & there is no check light…& lasts for about 30 seconds…If I get out & raise the hood…the sound has stopped. on 9.12.19 I had a check engine light with a PO411 error…my mechanic replaced the Air Check Valve at $800…wow…but the check light went out & all was good until this siren noise…started about a month ago…
What is your fix?
Pat…somewhere in the middle of California

What you might be hearing is the secondary air injection pump for the catalytic converter.

When these start to fail, they can sound like a siren.