It could well be that the stock is bending up and touch in the barrel... assuming the barrel was free-floating...
I had this with a Rem 700 SPS with a hogue stock. Without a bipod the barrel was free floating, you could pass paper all the way up to the reciever.
When i mounted a bipod on the front swivel the stock flexed upwards and touched the barrel... and if you put your hand on the comb of the stock and moved it left anf riht, you could see the barrel moving in the channel.
I ditched the Hogue stock and got the Choate "ultimate sniper" stock... very rigid ( but very heavy!!)