If it's a relay that's buzzing and mostly just under load at higher road speeds, not plastic vibrating, and if I had to bet, it would be a failing fuel pump relay. In 700/900s, it's either the tall white relay, 2nd row left in front of the round red bulb out sensor relay, or in later B230Fs it can be the small cube relay, rightmost in the front row. Drive with your ash tray holder removed and simply touch the top of the ful pump relay when it's making noise. You may well feel it buzzing or the sound might change when being pushed down.

If that's it then later remove it, with minimal wiggle pulling it straight up using neighbouring relays for hand leverage, and carefully inspect under a bright light the relay case, the relay bottom, relay spade contacts, relay tray plastic socket and down into the female contacts for signs of overheat or anything looking less than pristeen. If it's the white fuel pump relay, then pry off the case and again look for signs of hot contact areas inside, also using a magnifying glass see if you can see pitting on the relay coil contact faces. At the age of these cars, the fuel pump relay can be considered a wear item. Worn fuel pumps can start to overheat what would otherwise be an okay relay and speed it's demise. Re-flowing the solder at the main joints can restore relays that have cracked and failed through repeated thermal expansion, but I can't imagine that being the cause of vibrations. The small cube fuel pump relays are basically non-repairable in my experience.

Another distinct possibilty is plastic vibrating in the lower console area, such as the relay tray in it's holder frame. Just having had your hand in the relay area may have momentarily shifted plastic conntact. Push down or in on the sides of the relay tray and frame to see if the sound changes. Note the little plastic fuse puller on the right could be hanging half in mid-air by it's plastic cord and the plastic vibrate against anything there. There's lots of other plastic, cables and wiring in that area that could also vibrate with road speed.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now