iPhone 4 reverb/echo with backcover on

Hi

have an iphone 4 where I recently changed the mainboard.

After reassambling everthing, my call partner told me, that they always hear their voice when we are on phone calls....Kind of an echo of themselves

After removing the mainboard and assembling it again, I realized, that the problem only occures, when the backcover is on the phone??

Did anybody ever witness this problem? or can anybody tell me if it's causes by the frame or if it is a mainboard issue???

does anybody have a solution for it....

thank you in advance