[sarex] FT-847 Problem

Hello all, Has anybody experienced instability with their FT-847 on 2 m SSB every so-often the RX breaks into a shrill chirping for maybe 10 -20 seconds at random, then OK for a week or two (perhaps!) The RF amp and ATT. buttons don't appear to have any effect on stopping it when the problem is there. It doesn't appear to have any real effect on sensitivity, as it will receive a SSB signal while the noise is there, it's just hard to listen too. Any help would be appreciated, the fault being so intermittent it's going to be hard to find. Please reply direct. Thanks. PS. Antenna on or off makes no difference