Rattling when morning start up on Altima

Rattling noise from my 2015 Altima for about 5 seconds every morning it just started a week ago i took it to dealership and asked if they can tighten heat shields or replace them but was told they can’t do that it all comes as one piece and I'd have to replace manifold and converter for $1100 that's crazy I don't have a check engine light or anything…please help I need some answers what can it be I need to fix it