Fang 55

I have a 2015 Avalon XLE with 57000 miles. For about 4 months now I am hearing an intermittent rattling sound coming from the aft right under the hood. It sounds like water boiling but the temperature gauge shows no sign of overheating. I thought it may be in the internal heating system but it too works as advertised. The worse part is the sound comes and goes but intermittently in all different driving and weather conditions. Has anyone out there have a similar problem? Dave Sonnenberg