Strange receiver issue "creaking door"

[Bengt]

Hi all,

Discovered a atrange sound when receiving some "weak" stations. After further investigation the "door" sound appears only at abt -90dBm signal level. Eighter lower or higher signal - the strange sound goes away. Sound example attached.

Settings as follows:
145.425 12,5kHz BW. SQ does not interfer.
Also at 70cm, there is a slight appearance, but main concern is on 2m.

Is there any schematics for the radio and/or any ideas on what is going on?

73

SM6TOB, Bengt

[Dan]

I have received some pretty strange sounds from intermod. If you open the squelch and sit on a nearby unused frequency do you ever hear it. Mine was intermittent and I finally traced it to a pager transmitter (yes pager in this day and age). Used a SDR dongle to finally trace it out and ended up making a cavity filter to kill it. Or maybe you have some kind of AGC instability at that signal level. Finally, I have a VHF receiver that will roar if it has a bad antenna impedance match, I assume your SWR is good. Try a choke right at the receiver input and some ferrite on the power cables as well. Good luck, this stuff can be a stinker.