Weird vibration when I let go off gas between 65-75 mph
 

Hello all,
I recently bought a used 09 IS 250 with only 52K miles but I noticed that the car vibrates for a few seconds when I let go off gas between 65-75 mph. Does anyone know what could be wrong with it?
I saw a post with this problem on Club Lexus back in 2011 car with 100K but there was not a resolution for the post. Thanks in advance for any help or advice.